NCERT Class XI Mathematics - Straight Lines - Solutions

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Question : 46
Total: 74
Two lines passing through the point (2, 3) intersects each other at an angle of 60°. If slope of one line is 2, find equation of the other line.
Solution:  
We have given a point (2, 3), through which two lines are passing and intersects at an angle of 60°.
Let m be the slope of the other line
⇒ tan 60° = |
m2
1+2m
|
⇒ ± 3 =
m2
1+2m

3 =
m2
1+2m
... (i)
or - 3 =
m2
1+2m
... (ii)
3+23m = m - 2 or 3+23m = 2 - m
3 + 2 = m - 23 m or 23 m + m = 2 - 3
⇒ m =
3+2
123
or m =
23
1+23

Since the line passes through (2, 3)
∴ Its equation is either
y - 3 =
3+2
123
(x2)
... (iii)
or y - 3 =
23
1+23
(x2)
... (iv)
(iii) ⇒ (y − 3)(1 − 23) = (3 + 2) (x − 2)
⇒ y (1 − 23) − 3(1 − 23) = (3 + 2)x − 2(3 + 2)
⇒ y (1 − 23) − (3 − 6 3) = (3 + 2)x − 2 3 −4
⇒ 2 3 + 4 − 3 + 6 3 =(3 + 2)x +(23 − 1)y
⇒(3 + 2)x + (23 − 1)y− 8 3 − 1 = 0 ... (v)
And
(iv) ⇒ (y − 3) (1 + 23) = (2 − 3) (x −2)
⇒ y(1 + 23) − 3 (1 + 23) = (2 − 3)x − 2(2 − 3)
⇒ − 3 − 6 3 + 2(2 − 3) = (2 − 3)x− y(1+ 2 3)
⇒ − 3 − 6 3 +4 − 2 3 =(2 − 3)x − y (1 + 2 3)
⇒ (2 − 3)x −(1 + 2 3)y + 83 − 1 = 0 ... (vi)
Thus (v) and (vi) are the required equations of the line.
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