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° =|
| ⇒ ± √ 3 =
⇒√ 3 =
... (i)
or -√ 3 =
... (ii)
⇒√ 3 + 2 √ 3 m = m - 2 or √ 3 + 2 √ 3 m = 2 - m
⇒√ 3 + 2 = m - 2 √ 3 m or 2 √ 3 m + m = 2 - √ 3
⇒ m =
or m =
Since the line passes through (2, 3)
∴ Its equation is either
y - 3 =
( x − 2 ) ... (iii)
or y - 3 =
( x − 2 ) ... (iv)
(iii) ⇒ (y − 3)(1 − 2√ 3 ) = (√ 3 + 2) (x − 2)
⇒ y (1 − 2√ 3 ) − 3(1 − 2√ 3 ) = (√ 3 + 2)x − 2(√ 3 + 2)
⇒ y (1 − 2√ 3 ) − (3 − 6 √ 3 ) = (√ 3 + 2)x − 2 √ 3 −4
⇒ 2√ 3 + 4 − 3 + 6 √ 3 =(√ 3 + 2)x +(2√ 3 − 1)y
⇒(√ 3 + 2)x + (2√ 3 − 1)y− 8 √ 3 − 1 = 0 ... (v)
And
(iv) ⇒ (y − 3) (1 + 2√ 3 ) = (2 − √ 3 ) (x −2)
⇒ y(1 + 2√ 3 ) − 3 (1 + 2√ 3 ) = (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 + 8√ 3 − 1 = 0 ... (vi)
Thus (v) and (vi) are the required equations of the line.
Let m be the slope of the other line
⇒ tan 60° =
⇒
or -
⇒
⇒
⇒ m =
Since the line passes through (2, 3)
∴ Its equation is either
y - 3 =
or y - 3 =
(iii) ⇒ (y − 3)(1 − 2
⇒ y (1 − 2
⇒ y (1 − 2
⇒ 2
⇒(
And
(iv) ⇒ (y − 3) (1 + 2
⇒ y(1 + 2
⇒ − 3 − 6
⇒ − 3 − 6
⇒ (2 −
Thus (v) and (vi) are the required equations of the line.
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