NCERT Solutions Class 11 Maths Chapter 9 - Straight Lines

Q1: Write the equations for the x and y-axes.
Ans: The y-coordinate of every point on the x-axis is 0.
Therefore, the equation of the x-axis is y = 0.
The x-coordinate of every point on the y-axis is 0.
Therefore, the equation of the y-axis is y = 0.

Q2: Find the equation of the line which passes through the point (–4, 3) with slope .
Ans: We know that the equation of the line passing through point  , whose slope is m, is  .
Thus, the equation of the line passing through point (–4, 3), whose slope is  , is

Q3: Find the equation of the line which passes though (0, 0) with slope m.
Ans: We know that the equation of the line passing through point  , whose slope is m, is  .
Thus, the equation of the line passing through point (0, 0), whose slope is m,is
(y – 0) = m(x – 0)
i.e., y = mx

Q4: Find the equation of the line which passes though   and is inclined with the x-axis at an angle of 75°.
Ans: The slope of the line that inclines with the x-axis at an angle of 75° is m = tan 75°

We know that the equation of the line passing through point  , whose slope is m, is  .
Thus, if a line passes though  and inclines with the x-axis at an angle of 75°, then the equation of the line is given as

Q5: Find the equation of the line which intersects the x-axis at a distance of 3 units to the left of origin with slope –2.
Ans: It is known that if a line with slope m makes x-intercept d, then the equation of the line is given as
y = m(x – d)
For the line intersecting the x-axis at a distance of 3 units to the left of the origin, d = –3.
The slope of the line is given as m = –2
Thus, the required equation of the given line is
y = –2 [x – (–3)]
y = –2x – 6
i.e., 2x +  y + 6 = 0

Q6: Find the equation of the line which intersects the y-axis at a distance of 2 units above the origin and makes an angle of 30° with the positive direction of the x-axis.
Ans: It is known that if a line with slope m makes y-intercept c, then the equation of the line is given as
y = mx + c
Here, c = 2 and m = tan 30°  .
Thus, the required equation of the given line is

Q7: Find the equation of the line which passes through the points (–1, 1) and (2, –4).
Ans: t is known that the equation of the line passing through points (x₁, y₁) and (x₂, y₂) is  .
Therefore, the equation of the line passing through the points (–1, 1) and
(2, –4) is

Q8: The vertices of ΔPQR are P (2, 1), Q (–2, 3) and R (4, 5). Find equation of the median through the vertex R.
Ans: It is given that the vertices of ΔPQR are P (2, 1), Q (–2, 3), and R (4, 5).
Let RL be the median through vertex R.
Accordingly, L is the mid-point of PQ.
By mid-point formula, the coordinates of point L are given by
It is known that the equation of the line passing through points (x₁, y₁) and (x₂, y₂) is  .
Therefore, the equation of RL can be determined by substituting (x₁, y₁) = (4, 5) and (x₂, y₂) = (0, 2).

Hence, 

Thus, the required equation of the median through vertex R is  .

Q9: Find the equation of the line passing through (–3, 5) and perpendicular to the line through the points (2, 5) and (–3, 6).
Ans: The slope of the line joining the points (2, 5) and (–3, 6) is
We know that two non-vertical lines are perpendicular to each other if and only if their slopes are negative reciprocals of each other.
Therefore, slope of the line perpendicular to the line through the points (2, 5) and (–3, 6)  
Now, the equation of the line passing through point (–3, 5), whose slope is 5, is

Q10: A line perpendicular to the line segment joining the points (1, 0) and (2, 3) divides it in the ratio 1:n. Find the equation of the line.
Ans: According to the section formula, the coordinates of the point that divides the line segment joining the points (1, 0) and (2, 3) in the ratio 1: n is given by

The slope of the line joining the points (1, 0) and (2, 3) is

We know that two non-vertical lines are perpendicular to each other if and only if their slopes are negative reciprocals of each other.
Therefore, slope of the line that is perpendicular to the line joining the points (1, 0) and (2, 3)  
Now, the equation of the line passing through   and whose slope is  is given by

Q11: Find the equation of a line that cuts off equal intercepts on the coordinate axes and passes through the point (2, 3).
Ans: The equation of a line in the intercept form is

Here, a and b are the intercepts on x and y axes respectively.
It is given that the line cuts off equal intercepts on both the axes. This means that a = b.
Accordingly, equation (i) reduces to

Since the given line passes through point (2, 3), equation (ii) reduces to
2 + 3 = a ⇒ a = 5
On substituting the value of a in equation (ii), we obtain
x  + y = 5, which is the required equation of the line

Q12: Find equation of the line passing through the point (2, 2) and cutting off intercepts on the axes whose sum is 9.
Ans: The equation of a line in the intercept form is

Here, a and b are the intercepts on x and y axes respectively.
It is given that a  + b = 9 ⇒ b = 9 – a … (ii)
From equations (i) and (ii), we obtain

It is given that the line passes through point (2, 2). Therefore, equation (iii) reduces to

If a = 6 and b = 9 – 6 = 3, then the equation of the line is

If a = 3 and b = 9 – 3 = 6, then the equation of the line is

Q13: Find equation of the line through the point (0, 2) making an angle   with the positive x-axis. Also, find the equation of line parallel to it and crossing the y-axis at a distance of 2 units below the origin.
Ans: The slope of the line making an angle  with the positive x-axis is  
Now, the equation of the line passing through point (0, 2) and having a slope    is  .

The slope of line parallel to line   is  .
It is given that the line parallel to line  crosses the y-axis 2 units below the origin i.e., it passes through point (0, –2).
Hence, the equation of the line passing through point (0, –2) and having a slope   is

Q14: The perpendicular from the origin to a line meets it at the point (– 2, 9), find the equation of the line.
Ans: The slope of the line joining the origin (0, 0) and point (–2, 9) is  
Accordingly, the slope of the line perpendicular to the line joining the origin and point (– 2, 9) is

Now, the equation of the line passing through point (–2, 9) and having a slope m₂ is

Q15: The length L (in centimetre) of a copper rod is a linear function of its Celsius temperature C. In an experiment, if L = 124.942 when C = 20 and L = 125.134 when C = 110, express L in terms of C.
Ans: It is given that when C = 20, the value of L is 124.942, whereas when C = 110, the value of L is 125.134.
Accordingly, points (20, 124.942) and (110, 125.134) satisfy the linear relation between L and C.
Now, assuming C along the x-axis and L along the y-axis, we have two points i.e., (20, 124.942) and (110, 125.134) in the XY plane.
Therefore, the linear relation between L and C is the equation of the line passing through points (20, 124.942) and (110, 125.134).
(L – 124.942) =  

Q16: The owner of a milk store finds that, he can sell 980 litres of milk each week at Rs 14/litre and 1220 litres of milk each week at Rs 16/litre. Assuming a linear relationship between selling price and demand, how many litres could he sell weekly at Rs 17/litre?
Ans: The relationship between selling price and demand is linear.
Assuming selling price per litre along the x-axis and demand along the y-axis, we have two points i.e., (14, 980) and (16, 1220) in the XY plane that satisfy the linear relationship between selling price and demand.
Therefore, the linear relationship between selling price per litre and demand is the equation of the line passing through points (14, 980) and (16, 1220).

When x = Rs 17/litre,

Thus, the owner of the milk store could sell 1340 litres of milk weekly at Rs 17/litre.

Q17: P (a, b) is the mid-point of a line segment between axes. Show that equation of the line is 
Ans: Let AB be the line segment between the axes and let P (a, b) be its mid-point.

Let the coordinates of A and B be (0, y) and (x, 0) respectively.
Since P (a, b) is the mid-point of AB,

Thus, the respective coordinates of A and B are (0, 2b) and (2a, 0).
The equation of the line passing through points (0, 2b) and (2a, 0) is

On dividing both sides by ab, we obtain

Thus,the equation of the line is  .

Q18: Point R (h, k) divides a line segment between the axes in the ratio 1:2. Find equation of the line.
Ans: Let AB be the line segment between the axes such that point R (h, k) divides AB in the ratio 1: 2.
Let the respective coordinates of A and B be (x, 0) and (0, y).
Since point R (h, k) divides AB in the ratio 1: 2, according to the section formula,

Therefore, the respective coordinates of A and B are   and (0, 3k).
Now, the equation of line AB passing through points    and (0, 3k) is

Thus, the required equation of the line is 2kx + hy = 3hk.

Q19: By using the concept of equation of a line, prove that the three points (3, 0), (–2, –2) and (8, 2) are collinear.
Ans: In order to show that points (3, 0), (–2, –2), and (8, 2) are collinear, it suffices to show that the line passing through points (3, 0) and (–2, –2) also passes through point (8, 2).
The equation of the line passing through points (3, 0) and (–2, –2) is

It is observed that at x = 8 and y = 2,
L.H.S. = 2 × 8 – 5 × 2 = 16 – 10 = 6 = R.H.S.
Therefore, the line passing through points (3, 0) and (–2, –2) also passes through point (8, 2). Hence, points (3, 0), (–2, –2), and (8, 2) are collinear.