Equations CA Foundation Notes | EduRev

Quantitative Aptitude for CA CPT

Created by: Wizius Careers

CA Foundation : Equations CA Foundation Notes | EduRev

The document Equations CA Foundation Notes | EduRev is a part of the CA Foundation Course Quantitative Aptitude for CA CPT.
All you need of CA Foundation at this link: CA Foundation

EQUATIONS

LEARNING OBJECTIVES
After studying this chapter, you will be able to:

  • Understand the concept of equations and its various degrees – linear, simultaneous, quadratic and cubic equations;
  • Know how to solve the different equations using different methods of solution; and
  • Know how to apply equations in co-ordinate geometry.

2.1 INTRODUCTION

Equation is defined to be a mathematical statement of equality. If the equality is true for certain value of the variable involved, the equation is often called a conditional equation and equality sign ‘=’ is used; while if the equality is true for all values of the variable involved, the equation is called an identity.

Equations CA Foundation Notes | EduRev

is an identity since it holds for all values of the variable x.
Determination of value of the variable which satisfies an equation is called solution of the equation or root of the equation. An equation in which highest power of the variable is 1 is called a Linear (or a simple) equation. This is also called the equation of degree 1. Two or more linear equations involving two or more variables are called Simultaneous Linear Equations. An equation of degree 2 (highest Power of the variable is 2) is called Quadratic equation and the equation of degree 3 is called Cubic Equation.

For Example: 8x+17(x–3) = 4 (4x–9) + 12 is a Linear equation
3x2 + 5x +6 = 0 is a quadratic equation.
4x3 + 3x2 + x–7 = 1 is a Cubic equation.
x+2y = 12x+3y = 2 are jointly called simultaneous equations.

2.2 SIMPLE EQUATION

A simple equation in one unknown x is in the form ax + b = 0.
Where a, b are known constants and a10
Note: A simple equation has only one root.

Equations CA Foundation Notes | EduRev

Solution: By transposing the variables in one side and the constants in other side we have
Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Illustrations: 

1 . The denominator of a fraction exceeds the numerator by 5 and if 3 be added to both the
fraction becomes  Equations CA Foundation Notes | EduRev Find the fraction

Let x be the numerator and the fraction be  Equations CA Foundation Notes | EduRev. By the question  Equations CA Foundation Notes | EduRev 4x+12 = 3x+24 or x = 12

The required fraction is  Equations CA Foundation Notes | EduRev

2. If thrice of A’s age 6 years ago be subtracted from twice his present age, the result would be equal to his present age. Find A’s present age.

Let x years be A’s present age. By the question
2x–3(x–6) = x
or 2x–3x+18 = x
or –x+18 = x
or 2x = 18
or x=9
∴ A’s present age is 9 years.

3. A number consists of two digits the digit in the ten’s place is twice the digit in the unit’s place. If 18 be subtracted from the number the digits are reversed. Find the number.
Let x be the digit in the unit’s place. So the digit in the ten’s place is 2x. Thus the number becomes 10(2x)+x. By the question

20x+x–18 = 10x + 2x
or 21x–18 = 12x
or 9x = 18
or x = 2
So the required number is 10 (2 × 2) + 2 = 42.

4. For a certain commodity the demand equation giving demand ‘d’ in kg, for a price ‘p’ in rupees per kg. is d = 100 (10 – p). The supply equation giving the supply s in kg. for a price p in rupees per kg. is s = 75(p - 3). The market price is such at which demand equals supply. Find the market price and quantity that will be bought and sold.
Given d = 100(10 – p) and s = 75(p – 3).
Since the market price is such that demand (d) = supply (s) we have
100 (10 – p) = 75 (p – 3) or 1000 – 100p = 75p – 225
Equations CA Foundation Notes | EduRev

So market price of the commodity is Rs. 7 per kg.
∴ the required quantity bought = 100 (10 – 7) = 300 kg.
and the quantity sold = 75 (7 – 3) = 300 kg.

2.3 SIMULTANEOUS LINEAR EQUATIONS IN TWO UNKNOWNS

The general form of a linear equations in two unknowns x and y is ax + by + c = 0 where a b are non-zero coefficients and c is a constant. Two such equations a1x + b1y + c1 = 0 and a2 x + b2 x + c2 = 0 form a pair of simultaneous equations in x and y. A value for each unknown which satisfies simultaneously both the equations will give the roots of the equations.

2.4 METHOD OF SOLUTION

1. Elimination Method: In this method two given linear equations are reduced to a linear equation in one unknown by eliminating one of the unknowns and then solving for the other unknown.
Example 1: Solve: 2x + 5y = 9 and 3x – y = 5.

Solution: 2x + 5y = 9 …….. (i)
3x – y = 5 ………(ii)
By making (i) x 1,  2x + 5y = 9
and by making (ii) x 5, 15x – 5y = 25
__________________________________
Adding 17x = 34 or x = 2. Substituting this values of x in (i) i.e. 5y = 9 – 2x 

we find,
5y = 9 – 4 = 5 

∴y = 1 

∴x = 2, y = 1.

2. Cross Multiplication Method: Let two equations be:

a1x + b1y + c1 = 0
a2x + b2y + c2 = 0

We write the coefficients of x, y and constant terms and two more columns by repeating the coefficients of x and y as follows:

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Example 2: 3x + 2y + 17 = 0
5x – 6y – 9 = 0

Solution: 3x + 2y + 17 = 0 ....... (i)
5x – 6y – 9 = 0 ........(ii)
Method of elimination: By (i) x3 we get 9x + 6y + 51 = 0 ...... (iii)
Adding (ii) & (iii) we get 14x + 42 = 0

Equations CA Foundation Notes | EduRev

Putting x = –3 in (i) we get 3(–3) + 2y + 17 = 0
or, 2y + 8 = 0 or, y =  Equations CA Foundation Notes | EduRev

So  x = –3  and  y = –4
Method of cross-multiplication: 3x + 2y + 17 = 0
5x – 6y – 9 = 0

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

2.5 METHOD OF SOLVING SIMULTANEOUS LINEAR EQUATION WITH THREE VARIABLES

Example 1: Solve for x, y and z:
2x–y + z = 3 x + 3y – 2z = 11 3x – 2y + 4z = 1

Solution: (a) Method of elimination
2x – y + z = 3 .......(i)
x + 3y – 2z = 11 .... (ii)
3x – 2y + 4z = 1 .... (iii)
By (i) × 2 we get
4x – 2y + 2z = 6 …. (iv)
By (ii) + (iv), 5x + y = 17 ….(v) [the variable z is thus eliminated]x
By (ii) × 2,  2x + 6y – 4z = 22 ….(vi)
By (iii) + (vi), 5x + 4y = 23 ....(vii)
By (v) – (vii), –3y = – 6 or y = 2
Putting y = 2 in (v) 5x + 2 = 17, or  5x = 15 or,  x = 3
Putting x = 3 and y = 2 in (i)
2 × 3 – 2 + z = 3
or 6–2+z = 3
or 4+z = 3
or z = –1
So x = 3, y = 2, z = –1 is the required solution.
(Any two of 3 equations can be chosen for elimination of one of the variables)

(b) Method of cross multiplication

We write the equations as follows:
2x – y + (z – 3) = 0
x + 3y + (–2z –11) = 0
By cross multiplication

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Substituting above values for x and y in equation (iii) i.e. 3x - 2y + yz = 1, 

we have
Equations CA Foundation Notes | EduRev

or 60–3z–10z–38 + 28z = 7
or 15z = 7–22 or 15z = –15 or z = –1
Equations CA Foundation Notes | EduRev

Thus x = 3, y = 2, z = –1
Example 2: Solve for x, y and z :

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

u+v+w = 5 ........ (i)
2u–3v–4w = –11........ (ii)
3u+2v–w = –6 ........ (iii)
By (i) + (iii) 4u+3v = –1 ........ (iv)
By (iii) x 4 12u+8v–4w = –24 ......... (v)
By (ii) – (v) –10u–11v = 13
or 10u + 11v = –13 .......... (vi)
By (iv) × 11 44x+33v = –11 …..…(vii)
By (vi) × 3 30u + 33v = –39 ……..(viii)
By (vii) – (viii) 14u = 28 or u = 2
Putting u = 2 in (iv) 4 × 2 + 3v = –1
or 8 + 3v = –1
or 3v = –9 or v = –3
Putting u = 2, v = –3 in (i) or 2–3 + w = 5
or –1 + w = 5 or w = 5+1 or w = 6

Equations CA Foundation Notes | EduRev

Example 3:   Solve for x y and z:

Equations CA Foundation Notes | EduRev

Solution: We can write as
Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

2.6 PROBLEMS LEADING TO SIMULTANEOUS EQUATIONS

Illustrations :
1. If the numerator of a fraction is increased by 2 and the denominator by 1 it becomes 1.
Again if the numerator is decreased by 4 and the denominator by 2 it becomes 1/2 . Find the fraction

Solution: Let x/y be the required fraction.

Equations CA Foundation Notes | EduRev

Thus x + 2 = y + 1 or x – y = –1 ......... (i)
and 2x – 8 = y–2 or 2x – y = 6 ......... (ii)
By (i) – (ii) –x = –7 or x = 7
from (i) 7–y = –1 or y = 8
So the required fraction is 7/8.

2. The age of a man is three times the sum of the ages of his two sons and 5 years hence his age will be double the sum of their ages. Find the present age of the man?

Solution: Let x years be the present age of the man and sum of the present ages of the two sons be y years.
By the condition x = 3y .......... (i)
and x + 5 = 2 ( y+5+5) ..........(ii)
From (i) & (ii) 3y + 5 = 2 (y+10)
or 3y + 5 = 2y + 20
or 3y – 2y = 20 – 5
or y = 15
∴ x = 3 × y = 3 × 15 = 45
Hence the present age of the main is 45 years

3. A number consist of three digit of which the middle one is zero and the sum of the other digits is 9. The number formed by interchanging the first and third digits is more than the original number by 297 find the number.

Solution: Let the number be 100x + y. we have x + y = 9……(i)
Also 100y + x = 100x + y + 297 …………………………….. (ii)
From (ii) 99(x – y) = –297
or x – y = –3 ….……………………………………………… (iii)

Adding (i) and (ii) 2x = 6 ∴ x = 3 ∴ from (i) y = 6
∴ Hence the number is 306.

2.7 QUADRATIC EQUATION

An equation of the form ax2 + bx + c = 0 where x is a variable and a, b, c are constants with a ≠ 0 is called a quadratic equation or equation of the second degree.
When b=0 the equation is called a pure quadratic equation; when b ¹ 0 the equation is called an adfected quadratic.
Examples: i) 2x2 + 3x + 5 = 0
ii) x– x = 0
iii) 5x2 – 6x –3 = 0
The value of the variable say x is called the root of the equation. A quadratic equation has got two roots.
How to find out the roots of a quadratic equation:
ax+ bx +c = 0 (a ≠ 0)
Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Let one root be and the other root be β

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

2.8 HOW TO CONSTRUCT A QUADRATIC EQUATION

For the equation ax2 + bx + c = 0 we have

Equations CA Foundation Notes | EduRev

or x2 – (Sum of the roots) x + Product of the roots = 0

2.9 NATURE OF THE ROOTS

Equations CA Foundation Notes | EduRev

i) If b2>4ac = 0 the roots are real and equal;
ii) If b2>4ac >0 then the roots are real and unequal (or distinct);
iii) If b2>4ac <0 then the roots are imaginary;
iv) If b2>4ac is a perfect square ( ≠ 0) the roots are real, rational and unequal (distinct);
v) If b2>4ac > 0 but not a perfect square the rots are real, irrational and unequal.
Since b2 – 4ac discriminates the roots b2 – 4ac is called the discriminant in the equation ax2 + bx + c = 0 as it actually discriminates between the roots.

Note: (a) Irrational roots occur in pairs that is if  Equations CA Foundation Notes | EduRev is a root then  Equations CA Foundation Notes | EduRev  is the other root of the same equation.

(b) If one root is reciprocal to the other root then their product is 1 and so  Equations CA Foundation Notes | EduRev = 1 i.e. c = a

(c) If one root is equal to other root but opposite in sign then. their sum = 0 and so  Equations CA Foundation Notes | EduRev = 0. i.e. b = 0. 

Example 1 : Solve x2 – 5x + 6 = 0

Solution: 1st method : x2 – 5x + 6 = 0
or x2 –2x –3x +6 = 0
or x(x–2) – 3(x–2) = 0
or (x–2) (x–3) = 0
or x = 2 or 3
2nd method (By formula) x2 – 5x + 6 = 0

Here a = 1 b = –5   c = 6 (comparing the equation with ax2 + bx+c = 0)

Equations CA Foundation Notes | EduRev

Example 2: Examine the nature of the roots of the following equations.
i) x2 – 8x2 + 16 = 0
ii) 3x2 – 8x + 4 = 0
iii) 5x2 – 4x + 2 = 0
iv) 2x2 – 6x – 3 = 0

Solution: (i) a = 1 b = –8 c = 16
b– 4ac = (–8)2 – 4.1.16 = 64 – 64 = 0
The roots are real and equal.
(ii) 3x2 – 8x + 4 = 0
a = 3 b = –8 c = 4
b2 – 4ac = (–8)2 – 4.3.4 = 64 – 48 = 16 > 0 and a perfect square
The roots are real, rational and unequal

iii) 5x2 – 4x + 2 = 0
b2 – 4ac = (–4)2 – 4.5.2 = 16–40 = –24 < 0
The roots are imaginary and unequal
(iv) 2x– 6x – 3 = 0
b2 – 4ac = (–6)2 – 4.2 (–3)
= 36 + 24 = 60 > 0
The root are real and unequal. Since b2 – 4ac is not a perfect square the roots are real irrational and unequal.

Illustrations:

1. If α and ß be the roots of x2 + 7x + 12 = 0 find the equation whose roots are ( α + ß )2 and (α - ß)2.

Solution : Now sum of the roots of the required equation
= (α + b)2 + (α + b)2 = (-7)2 + (α + b)2 - 4 ∝ b
= 49 + (–7)2 – 4x12 = 49 + 49 – 48 = 50

Product of the roots of the required equation = (α + b)2  (α - b)= 49 (49–48) = 49
Hence the required equation is x2 – (sum of the roots) x + product of the roots = 0
or x2 – 50x + 49 = 0

2. If a ,b be the roots of 2x2 – 4x – 1 = 0 find the value of  Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

or 4p+ 8p – 45 = 0
or 4p+ 18p – 10p – 45 = 0
or 2p(2p + 9) – 5(2p + 9) = 0
or (2p – 5) (2p + 9) = 0.

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

or t2 + 32 = 12t
or t2 – 12t + 32 = 0
or t2 – 8t – 4t + 32 = 0
or t(t–8) – 4(t–8) = 0
or (t–4) (t–8) = 0
∴ t = 48
For t = 4   2x = 4 = 22 i.e. x = 2
For t = 8   2x = 8 = 23 i.e. x = 3

Equations CA Foundation Notes | EduRev

∴ Required equation is : x2 – (sum of roots)x + (product of roots) = 0 or x2 – 4x + 1 = 0.

7. If α β are the two roots of the equation x2 – px + q = 0 form the equation whose roots are  Equations CA Foundation Notes | EduRev   

Solution: As α, β are the roots of the equation x2 – px + q = 0 α + β = – (– p) = p and α β = q.

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

or qx2 – (p2 – 2q) x + q = 0
8. If the roots of the equation p(q – r)x2 + q(r – p)x + r(p – q) = 0
are equal show that  Equations CA Foundation Notes | EduRev

Solution: Since the roots of the given equation are equal the discriminant must be zero ie. q2(r – p)2 – 4. p(q – r) r(p – q) = 0

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

2.10 PROBLEMS ON QUADRATIC EQUATION

1. Difference between a number and its positive square root is 12; find the numbers?

Solution: Let the number be x.
Then Equations CA Foundation Notes | EduRev = 12 …………… (i)

Equations CA Foundation Notes | EduRev

or (y – 4) (y + 3) = 0 ∴ Either y = 4 or y = – 3 i.e. Either  Equations CA Foundation Notes | EduRev or  Equations CA Foundation Notes | EduRev

If Equations CA Foundation Notes | EduRev x = 9 if does not satisfy equation (i) so Equations CA Foundation Notes | EduRevor x=16.

2. A piece of iron rod costs Rs. 60. If the rod was 2 metre shorter and each metre costs Re 1.00 more, the cost would remain unchanged. What is the length of the rod?
Solution: Let the length of the rod be x metres. The rate per meter is Rs. Equations CA Foundation Notes | EduRev

New Length = (x – 2); as the cost remain the same the new rate per meter is Equations CA Foundation Notes | EduRev 

Equations CA Foundation Notes | EduRev

or x– 2x = 120 or x2 – 2x – 120 = 0 or (x – 12) (x + 10) = 0.
Either x = 12 or x = –10 (not possible) ∴ Hence the required length = 12m.

3. Divide 25 into two parts so that sum of their reciprocals is 1/6.
Solution: let the parts be x and 25 – x

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

or x(x–15) – 10(x–15) = 0
or (x–15) (x–10) = 0

or x = 10, 15
So the parts of 25 are 10 and 15.

2.11 SOLUTION OF CUBIC EQUATION

On trial basis putting some value of x to check whether LHS is zero then to get a factor.
This is a trial and error method. With this factor to factorise the LHS and then to get values of x .

Illustrations :

1. Solve x3 – 7x + 6 = 0
Putting x = 1 L.H.S is Zero. So (x–1) is a factor of x3 – 7x + 6
We write x3–7x +6 = 0 in such a way that (x–1) becomes its factor. This can be achieved by writing the equation in the following form.

or x3–x2+x2–x–6x+6 = 0
or x2(x–1) + x(x–1) – 6(x–1) = 0
or (x–1)(x2+x–6) = 0
or (x–1)(x2+3x–2x–6) = 0
or (x–1){x(x+3) – 2(x+3) } = 0
or (x–1)(x–2)(x+3) = 0

2. Solve for real x: x+ x + 2 = 0

Solution: By trial we find that x = –1 makes the LHS zero. So (x + 1) is a factor of x3 + x + 2
We write x3 + x + 2 = 0 as x+ x2 – x2 – x + 2x + 2 = 0
or x2(x + 1) – x(x + 1) + 2(x + 1) = 0
or (x + 1) (x2 – x + 2) = 0.
Either x + 1 = 0
or x2 – x + 2 = 0 i.e. x = –1
Equations CA Foundation Notes | EduRev

As x =  Equations CA Foundation Notes | EduRev is not real, x = –1 is the required solution.

2.12 APPLICATION OF EQUATIONS IN CO–ORDINATE GEOMETRY

Introduction: Co-ordinate geometry is that branch of mathematics which explains the problems of geometry with the help of algebra

Distance of a point from the origin.

Equations CA Foundation Notes | EduRev

P (x, y) is a point.
By Pythagoras’s Theorem OP2 =OL2 + PL2 or OP2 = x+ y2
So Distance OP of a point from the origin O is  Equations CA Foundation Notes | EduRev

Distance between two points

Equations CA Foundation Notes | EduRev

By Pythagoras’s Theorem PQ2=PT2 +QT2

Equations CA Foundation Notes | EduRev

So distance between two points (x1 y1) and (x2 y2) is given by  Equations CA Foundation Notes | EduRev

2.13 EQUATION OF A STRAIGHT LINE

Equations CA Foundation Notes | EduRev

(I) The equation to a straight line in simple form is generally written as y=mx+c …… (i)
where m is called the slope and c is a constant.

If P1(x1, y1) and P2(x2, y2) be any two points on the line the ratio  Equations CA Foundation Notes | EduRev is known as the
slope of the line.

We observe that B is a point on the line y = mx+c and OB is the length of the y-axis that is intercepted by the line and that for the point B x=0.
Substituting x=0 in y=mx+c we find y=c the intercept on the y axis.
This form of the straight line is known as slope–intercept form.

Note : (i) If the line passes through the origin (0, 0) the equation of the line becomes y = mx (or x=my)
(ii) If the line is parallel to x–axis, m=0 and the equation of the line becomes y = c (or x = b b is the intercept on x–axis)

(iii) If the line coincides with x–axis, m=0, c=0 then the equation of the line becomes y=0 which is the equation of x–axis. Similarly x=0 is the equation of y–axis.
(II) Let y = mx + c ………….. (i) be the equation of the line p1p2.

Let the line pass through (x1, y1). So we get
y= mx1+c ...(ii) By (i) – (ii) y–y= m(x–x1) … (iii)
which is another from of the equation of a line to be used when the slope(m) and any point (x1 y1) on the line be given. This form is called point–slope form.
(III) If the line above line (iii) passes through another point (x2, y2). we write y2–y1 = m(x2–x1)

Equations CA Foundation Notes | EduRev

Which is the equation of the line passing through two points (x1, y1) and (x2, y2)
(IV) We now consider a straight line that makes x-intercept = a and y-intercept = b
Slope of the line

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

If (x, y) is any point on this line we may also write the slope as 

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

The form  Equations CA Foundation Notes | EduRev  is called intercept form of the equation of the line and the same is to be used when x–intercept and y–intercept be given.

Note: (i) The equation of a line can also be written as ax+by+c = 0
(ii) If we write ax+by+c = 0 in the form y = mx+c

Equations CA Foundation Notes | EduRev

(iii) Two lines having slopes m1 and m2 are parallel to each other if and only if m1 = m2 and perpendicular to each other if and only if m1m2 = –1
(iv) Let ax + by + c  =  0 be a line. The equation of a line parallel to
ax + by + c = 0  is  ax + by + k = 0 and the equation of the line perpendicular to
ax + by + c = 0  is  bx– ay +k = 0
Let lines ax + by + c = 0 and a1x+b1y+c1 = 0 intersect each other at the point (x1, y1).
So ax1 + by1 + c = 0
a1x1 + b1y1 + c1 = 0

By cross multiplication  Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

Example : Let the lines 2x+3y+5 = 0 and 4x–5y+2 = 0 intersect at (x1 y1). To find the point of intersection we do cross multiplication as
2x1 + 3y1 + 5 = 0
4x1 + 5y1 + 2 = 0

Equations CA Foundation Notes | EduRev

Solving x1 =19/2 y1=–8

(V) The equation of a line passing through the point of intersection of the lines ax + by + c = 0 and a1x + b1y + c = 0 can be written as ax+by+c+K (a1x+b1y+c) = 0 when K is a constant.

(VI) The equation of a line joining the points (x1 y1) and (x2 y2) is given as

Equations CA Foundation Notes | EduRev

If any other point (x3 y3) lies on this line we get

Equations CA Foundation Notes | EduRev

which is the required condition of collinearity of three points.

Illustrations:

1. Show that the points A(2, 3) B(4, 1) and C(–2, 7) are collinear.
Solution : Using the rule derived in VI above we may conclude that the given points are collinear if 2(1–7)+4(7–3)–2(3–1)=0 i.e. if –12+16–4=0 which is true.
So the three given points are collinear

2. Find the equation of a line passing through the point (5, –4) and parallel to the line 4x+7y+5 = 0

Solution : Equation of the line parallel to 4x+7y+5 = 0 is 4x+7y+K = 0
Since it passes through the point (5, –4) we write
4(5) + 7(–4) + k = 0
or 20 – 28 + k = 0
or –8 + k = 0
or k = 8
The equation of the required line is therefore 4x+7y+8 = 0.

3. Find the equation of the straight line which passes through the point of intersection of the straight lines 2x+3y = 5 and 3x+5y = 7 and makes equal positive intercepts on the coordinate axes.

Solution: 2x+3y–5 = 0
3x+5y–7 = 0
By cross multiplication

Equations CA Foundation Notes | EduRev

So the point of intersection of the given lines is (4, –1)
Let the required equation of line be

Equations CA Foundation Notes | EduRev  (*for equal positive intercepts a=b)
∴ x + y = a
Since it passes through (4, –1) we get 4 – 1 = a or a = 3
The equation of the required line is therefore x + y = 3

4. Prove that (3, 1) (5, –5) and (–1, 13) are collinear and find the equation of the line through these three points.

Solution: If A (3, 1) B (5, – 5) and C (–1, 13) are collinear we may write
3(–5–13) +5(13–1) –1(1+5) = 0
or 3(–18) +5(12) – 6 = 0 which is true.
Hence the given three points are collinear.
As the points A, B, C are collinear, the required line will be the line through any of these two points. Let us find the equation of the line through B(5, – 5) and A (3, 1)
Using the rule derived in III earlier we find

Equations CA Foundation Notes | EduRev

or 3x + y = 10 is the required line.

5. Find the equation of the line parallel to the line joining points (7, 5) and (2, 9) and passing through the point (3, –4).

Solution : Equation of the line through the points (7, 5) and (2, 9) is given by

Equations CA Foundation Notes | EduRev

or –5y + 25 = 4x–28
or 4x+5y–53 = 0
Equation of the line parallel to 4x+5y–53=0 is 4x+5y+k = 0
If it passes through (3, –4) we have 12–20+k = 0 i.e. k=8
Thus the required line is 4x+5y+8 = 0

6. Prove that the lines 3x – 4y + 5 = 0, 7x – 8y + 5 = 0 and 4x + 5y = 45 are concurrent.

Solution: Let (x1 y1) be the point of intersection of the lines
3x – 4y +5 = 0 …………… (i)
7x – 8y + 5 = 0 …………... (ii)
Then we have 3x1 – 4y1 + 5 = 0
7x1 – 8y1 + 5 = 0

Equations CA Foundation Notes | EduRev

Hence (5, 5) is the point of intersection. Now for the line 4x + 5y = 45
we find 4.(5) + 5.5 = 45; hence (5, 5) satisfies the equation 4x+5y=45.
Thus the given three lines are concurrent.

7. A manufacturer produces 80 T.V. sets at a cost Rs. 220000 and 125 T.V. sets at a cost of Rs. 287500. Assuming the cost curve to be linear find the equation of the line and then use it to estimate the cost of 95 sets.

Solution: Since the cost curve is linear we consider cost curve as y = Ax + B where y is total cost. Now for x = 80 y = 220000.
 ∴ 220000 = 80A +B …..(i) and for x = 125 y=287500
∴ 287500 = 125A +B ……(ii)
Subtracting (i) from (ii) 45A = 67500 or A = 1500
From (i) 220000 – 1500 ´ 80 = B or B = 220000 – 120000 = 100000
Thus equation of cost line is y = 1500x + 100000.
For x = 95 y = 142500 + 100000 = Rs. 242500.
∴ Cost of 95 T.V. set will be Rs. 242500.

2.14 GRAPHICAL SOLUTION TO LINEAR EQUATIONS

1 . Drawing graphs of straight lines From the given equation we tabulate values of (x, y) at least 2 pairs of values and then plot them in the graph taking two perpendicular axis (x, y axis). Then joining the points we get the straight line representing the given equation.

Example 1 : Find the graph of the straight line having equation 3y = 9 – 2x

Solution: We have 2x + 3y = 9. We tabulate y =  Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev  

Equations CA Foundation Notes | EduRev

Here AB is the required straight line shown in the graph.

Example 2 : Draw graph of the straight lines 3x +4y = 10 and 2x – y = 0 and find the point of intersection of these lines.

Solution: For 3x + 4y = 10 we have y =  Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

For 2x – y = 0 we tabulate

Equations CA Foundation Notes | EduRev

Equations CA Foundation Notes | EduRev

From the graph, the point of intersection is (1, 2)

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