Straight Line Question 71

Question: In what direction a line be drawn through the point (1, 2) so that its points of intersection with the line $ x+y=4 $ is at a distance $ \frac{\sqrt{6}}{3} $ from the given point [IIT 1966; MNR 1987]

Options:

A) $ 30^{o} $

B) $ 45^{o} $

C) $ 60^{o} $

D) $ 75^{o} $

Show Answer

Answer:

Correct Answer: D

Solution:

  • Let the required line through the point (1,2) be inclined at an angle $ \theta $ to the axis of x. Then its equation is $ \frac{x-1}{\cos \theta }=\frac{y-2}{\sin \theta }=r $…..(i) where r is distance of any point (x, y) on the line from the point (1, 2). The coordinates of any point on the line (i) are $ (1+r\cos \theta ,2+r\sin \theta ) $ . If this point is at a distance $ \frac{\sqrt{6}}{3} $ form (1, 2), then $ r=\frac{\sqrt{6}}{3}. $ Therefore, the point is $ ( 1+\frac{\sqrt{6}}{3}\cos \theta ,\text{ }2+\frac{\sqrt{6}}{3}\sin \theta ) $ . But this point lies on the line $ x+y=4 $ .
    Þ $ \frac{\sqrt{6}}{3}(\cos \theta +\sin \theta )=1 $ or $ \sin \theta +\cos \theta =\frac{3}{\sqrt{6}} $
    Þ $ \frac{1}{\sqrt{2}}\sin \theta +\frac{1}{\sqrt{2}}\cos \theta =\frac{\sqrt{3}}{2} $ , {Dividing both sides by $ \sqrt{2} $ }
    Þ $ \sin (\theta +45^{o})=\sin 60^{o} $ or sin $ 120^{o} $
    Þ $ \theta =15^{o} $ or $ 75^{o} $ .