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All questions of Quadratic Equations for UPSC CSE Exam

The roots of the equation 3x2 - 12x + 10 = 0 are?
  • a)
    rational and unequal
  • b)
     complex
  • c)
    real and equal
  • d)
    irrational and unequal
  • e)
    rational and equal
Correct answer is option 'D'. Can you explain this answer?

The discriminant of the quadratic equation is (-12)2 - 4(3)(10) i.e., 24. As this is positive but not a perfect square, the roots are irrational and unequal.

The two numbers whose sum is 27 and their product is 182 are
  • a)
    12 and 13
  • b)
    12 and 15
  • c)
    14 and 15
  • d)
    13 and 14
Correct answer is option 'D'. Can you explain this answer?

Prateek Gupta answered
Explanation:Let the one number be xx .As the sum  of numbers is 27 , then the other number will be (27−x)(27−x)                                                                                                                                    According to question

The sum of the squares of two consecutive positive integers exceeds their product by 91. Find the integers?
  • a)
    9, 10
  • b)
     10, 11
  • c)
    11, 12
  • d)
    12, 13
  • e)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Dhruv Mehra answered
Let the two consecutive positive integers be x and x + 1
x2 + (x + 1)2 - x(x + 1) = 91
x2 + x - 90 = 0
(x + 10)(x - 9) = 0 => x = -10 or 9.
As x is positive x = 9
Hence the two consecutive positive integers are 9 and 10.

The product of two successive integral multiples of 5 is 1050. Then the numbers are
  • a)
    35 and 40
  • b)
    25 and 30
  • c)
    25 and 42
  • d)
    30 and 35
Correct answer is option 'D'. Can you explain this answer?

Prateek Gupta answered
Explanation:
Let one multiple of 5 be x then the next consecutive multiple of will be (x+5) According to question,
Then the number are 30 and 35.

I. a2 + 8a + 16 = 0,
II. b2 - 4b + 3 = 0 to solve both the equations to find the values of a and b?
  • a)
    If a < b
  • b)
    If a ≤ b
  • c)
    If the relationship between a and b cannot be established
  • d)
    If a > b
  • e)
    If a ≥ b
Correct answer is option 'A'. Can you explain this answer?

Sagar Sharma answered
I. To solve the equation a^2 + 8a + 16 = 0, we can use the quadratic formula:

a = (-b ± √(b^2 - 4ac)) / (2a)

In this case, a = 1, b = 8, and c = 16. Plugging these values into the quadratic formula:

a = (-8 ± √(8^2 - 4(1)(16))) / (2(1))

Simplifying:

a = (-8 ± √(64 - 64)) / 2

a = (-8 ± √0) / 2

a = -8 / 2

a = -4

So, the value of a is -4.

II. To solve the equation b^2 - 4b + 3 = 0, we can factorize it:

(b - 1)(b - 3) = 0

Setting each factor equal to zero:

b - 1 = 0 or b - 3 = 0

b = 1 or b = 3

So, the values of b are 1 and 3.

Therefore, the values of a and b are -4, 1, and 3.

For all x, x+ 2ax + (10 − 3a) > 0, then the interval in which a lies, is?
  • a)
    a < -5
  • b)
    a > 5
  • c)
    -5 < a < 2
  • d)
    2 < a < 5
  • e)
    a < -2
Correct answer is option 'C'. Can you explain this answer?

Rahul Mehta answered
In f(x) = ax2 + bx + c
When a > 0 and D < 0
Then f(x) is always positive.
x2 + 2ax + 10 − 3a > 0, ∀x ∈ R
⇒ D < 0
⇒ 4a2 − 4(10 − 3a) < 0
⇒ a2 + 3a − 10 < 0
⇒ (a+5)(a−2) < 0
⇒ a ∈ (−5,2)

If the roots of the equation 2x2 - 5x + b = 0 are in the ratio of 2:3, then find the value of b?
  • a)
    3
  • b)
    4
  • c)
    5
  • d)
    6
  • e)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Aarav Sharma answered
To find the value of b, we need to use the fact that the roots of the equation are in the ratio of 2:3.

Let's assume the roots of the equation are 2k and 3k, where k is a constant.

Using the sum and product of roots formulas, we can write the equation as follows:

Sum of roots: 2k + 3k = -(-5/2) = 5/2
Product of roots: (2k)(3k) = b/2

Simplifying the equations, we get:

5k = 5/2
6k^2 = b/2

Now, let's solve for k:

5k = 5/2
k = 1/2

Substituting the value of k in the second equation, we get:

6(1/2)^2 = b/2
6(1/4) = b/2
6/4 = b/2
3/2 = b/2
b = 3

Therefore, the value of b is 3, which corresponds to option A.

A man could buy a certain number of notebooks for Rs.300. If each notebook cost is Rs.5 more, he could have bought 10 notebooks less for the same amount. Find the price of each notebook?
  • a)
    10
  • b)
    8
  • c)
    15
  • d)
    7.50
  • e)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Nikita Singh answered
Explanation:
Let the price of each note book be Rs.x.
Let the number of note books which can be brought for Rs.300 each at a price of Rs.x be y.
Hence xy = 300
=> y = 300/x 
(x + 5)(y - 10) = 300 => xy + 5y - 10x - 50 = xy
=>5(300/x) - 10x - 50 = 0 => -150 + x2 + 5x = 0
multiplying both sides by -1/10x
=> x2 + 15x - 10x - 150 = 0
=> x(x + 15) - 10(x + 15) = 0
=> x = 10 or -15
As x>0, x = 10.

If the roots of the equation (a+ b2)x− 2b(a + c)x + (b2+c2) = 0 are equal then 
  • a)
    2b = ac
  • b)
    b= ac
  • c)
    b = 2ac/(a + c)
  • d)
    b = ac
  • e)
    b = 2ac
Correct answer is option 'B'. Can you explain this answer?

(a+ b2)x− 2b(a + c)x + (b2+c2) = 0
Roots are real and equal ∴ D = 0
D = b− 4ac = 0
⇒ [−2b(a+c)]− 4(a+ b2)(b+ c2) = 0
⇒ b2(a+ c+ 2ac) −(a2b2 + a2c2 + b4 + c2c2) = 0
⇒ b2a+ b2c+ 2acb− a2b− a2c− b4 − b2c2 = 0
⇒ 2acb− a2c− 2acb= 0
⇒ (b− ac)= 0
⇒ b2 = ac

One root of the quadratic equation x2 - 12x + a = 0, is thrice the other. Find the value of a?
  • a)
    29
  • b)
    -27
  • c)
    28
  • d)
    7
  • e)
    None of these
Correct answer is option 'E'. Can you explain this answer?

Surbhi Sen answered
Explanation:
Let the roots of the quadratic equation be x and 3x.
Sum of roots = -(-12) = 12
a + 3a = 4a = 12 => a = 3
Product of the roots = 3a2 = 3(3)2 = 27.

Find the quadratic equations whose roots are the reciprocals of the roots of 2x2 + 5x + 3 = 0?
  • a)
    3x2 + 5x - 2 = 0
  • b)
    3x2 + 5x + 2 = 0
  • c)
    3x2 - 5x + 2 = 0
  • d)
    3x2 - 5x - 2 = 0
  • e)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Yash Patel answered
Explanation:
The quadratic equation whose roots are reciprocal of 2x2 + 5x + 3 = 0 can be obtained by replacing x by 1/x.
Hence, 2(1/x)2 + 5(1/x) + 3 = 0
=> 3x2 + 5x + 2 = 0

If the roots of a quadratic equation are 20 and -7, then find the equation?
  • a)
    x2 + 13x - 140 = 0
  • b)
    x2 - 13x + 140 = 0
  • c)
    x2 - 13x - 140 = 0
  • d)
    x2 + 13x + 140 = 0
  • e)
    None of these
Correct answer is option 'C'. Can you explain this answer?

Explanation:
Any quadratic equation is of the form
x2 - (sum of the roots)x + (product of the roots) = 0 ---- (1)
where x is a real variable. As sum of the roots is 13 and product of the roots is -140, the quadratic equation with roots as 20 and -7 is: x2 - 13x - 140 = 0.

Find the value of a/b + b/a, if a and b are the roots of the quadratic equation x2 + 8x + 4 = 0?
  • a)
    15
  • b)
    14
  • c)
    24
  • d)
    26
  • e)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Manoj Ghosh answered
a/b + b/a = (a2 + b2)/ab = (a2 + b2 + a + b)/ab 
= [(a + b)2 - 2ab]/ab
a + b = -8/1 = -8
ab = 4/1 = 4
Hence a/b + b/a = [(-8)2 - 2(4)]/4 = 56/4 = 14.

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