Question
Class 11MathematicsStraight Lines

Find the equation of the lines through the point (3, 2) which make an angle of 45° with the line x – 2y = 3. 

Verified Answer

Let the slope of the required line be m1.

The given line can be written as y = 1/2x - 3/2, which is of the form y = mx + c.

∴ Slope of the given line = m2 = 1/2

It is given that the angle between the required line and line x - 2y - 3 =  is 45°.

We know that if θ is the acute angle between line l1 and l2 with slopes m1 and m2 respectively, then

tanθ = |m1 - m2/1 + m1m2|

∴ tan45° = |m1 - m2/1 + m1m2|

1 = |1/2 - m1/1 + m1/2| = |1 - 2m1/2 + m1|

1 = |1 - 2m1/2 + m1| ⇒ 1 = ±1 - 2m1/2 + m1

1 = 1 - 2m1/2 + m1 or 1 = -1 - 2m1/2 + m1

2 + m1 = 1 - 2m1 or 2 + m1 = -1 + 2m1

⇒ m1 = -1/3 or m1 = 3

Case I: m1 = 3

The equation of the line passing through (3, 2) and having a slope of 3 is:

y - 2 = 3(x - 3)

y - 2 = 3x - 9

3x - y = 7

Case II: m1 = -1/3

The equation of the line passing through (3, 2) and having a slope of -1/3 is:

y - 2 = -1/3 (x - 3)

3y - 6 = -x + 3

x + 3y = 9

Therefore, the equations of the lines are 3x - y = 7 and x + 3y = 9.