The angle between the lines $y^{2} \sin ^{2} \theta-x y \sin ^{2} \theta+x^{2}\left(\cos ^{2}…

The angle between the lines $y^{2} \sin ^{2} \theta-x y \sin ^{2} \theta+x^{2}\left(\cos ^{2} \theta-1\right)=0$ is
  1. $\frac{\pi}{4}$
  2. $\frac{\pi}{3}$
  3. $\frac{\pi}{6}$
  4. $\frac{\pi}{2}$

Solution

We have $y^{2} \sin ^{2} \theta-x y \sin ^{2} \theta+x^{2}\left(\cos ^{2} \theta-1\right)=0$ $\therefore\left(\sin ^{2} \theta\right) y^{2}-\left(\sin ^{2} \theta\right)(x y)-\left(\sin ^{2} \theta\right) x^{2}=0$ $\therefore\left(\sin ^{2} \theta\right)\left(y^{2}-x y-x^{2}\right)=0 \quad \Rightarrow \sin ^{2} \theta=0 \Rightarrow \theta=\frac{\pi}{2}$ Note : sum of coefficients of $x^{2}$ and $y^{2}$ is zero.

Asked in: MHT CET 2020 (20 Oct Shift 2)

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