NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 64
Total: 74
Find the equation of the lines through the point (3, 2) which make an angle of 45° with the line x – 2y = 3.
Solution:  
The given equation of a line x – 2y = 3.
∴ Slope of the given line is
1
2

Let m be the slope of the required line, then tan 45° = |
m
1
2
1+m(
1
2
)
|

⇒ 1 = |
2m1
2+m
|
⇒ ± 1 =
2m1
2+m

⇒ 2 + m = 2m – 1 or, – 2 – m = 2m – 1
⇒ 2 + 1 = 2m – m or, – 2 + 1 = 2m + m ⇒ 3 = m or, – 1 = 3m
⇒ m = 3 or, m =
1
3

Case (i) : If m = 3, then the equation of the line passing through (3, 2)
is y – 2 = 3(x – 3)
⇒ y – 2 = 3x – 9 ⇒ 3x – y + 2 – 9 = 0 ⇒ 3x – y – 7 = 0.
Case (ii) : If m =
1
3
, then the equation of the line passing through (3, 2) is
y - 2 =
1
3
(x - 3)
⇒ 3y – 6 = – x + 3 ⇒ x + 3y – 6 – 3 = 0 ⇒ x + 3y – 9 = 0.
∴ The required equation of lines are 3x – y – 7 = 0 and x + 3y – 9 = 0.
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