Straight Line Question 133

Question: Let L be the line $ 2x+y=2 $ . If the axes are rotated by $ 45^{o} $ , then the intercepts made by the line L on the new axes are respectively [Roorkee Qualifying 1998]

Options:

A) $ \sqrt{2} $ and 1

B)1 and $ \sqrt{2} $

C) $ 2\sqrt{2} $ and $ 2\sqrt{2}/3 $

D) $ 2\sqrt{2}/3 $ and $ 2\sqrt{2} $

Show Answer

Answer:

Correct Answer: C

Solution:

  • Suppose the axes are rotated in the anticlockwise direction through an angle $ 45^{o} $ . To find the equation of L w.r.t the new axis, we replace x by $ x\cos \alpha +y\sin \alpha $ and by $ x\sin \alpha -y\cos \alpha $ , so that equation of line w.r.t. new axes is Þ $ 1/1(x\cos 45^{o}-y\sin 45^{o})+\frac{1}{2}(x\sin 45^{o}+y\cos 45^{o})=1 $ Since, p, q are the intercepts made by the line on the coordinate axes. We have on putting (p, 0) and then (0, q) Þ $ \frac{1}{p}=\frac{1}{a}\cos \alpha +\frac{1}{b}\sin \alpha \Rightarrow \frac{1}{q}=-\frac{1}{a}\sin \alpha +\frac{1}{b}\cos \alpha $
    Þ $ \frac{1}{p}=\frac{1}{1}\cos 45^{\circ}+\frac{1}{2}\sin 45^{\circ} $

$ \Rightarrow \frac{1}{p}=\frac{1}{\sqrt{2}}+\frac{1}{2}.\frac{1}{\sqrt{2}}=\frac{3}{2\sqrt{2}} $ \ $ p=\frac{2\sqrt{2}}{3} $ ; \ $ \frac{1}{q}=-\frac{1}{1}\sin 45^{o}+\frac{1}{2}\cos 45^{o} $ $ \frac{1}{q}=\frac{-1}{\sqrt{2}}+\frac{1}{2\sqrt{2}}=-\frac{1}{2\sqrt{2}},\therefore q=2\sqrt{2} $ So intercept made by is assumed on the new axis $ ( 2\sqrt{2}/3,2\sqrt{2} ) $ . If the rotation is assumed in clockwise direction, so intercept made by the line on the new axes would be $ ( 2\sqrt{2},2\sqrt{2}/3 ) $ .



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