Q88 / 160 · TS EAMCET 2018 (07 May Shift 1) · Mathematics

If the lines represented by $x^2-2 h x y-y^2=0$ are rotated about $(0,0)$ through an angle $\alpha$ one in clockwise direction and the other in the counter clockwise direction, then the combined equation of the bisectors of the angle between the lines thus obtained is

  • A
    $x^2-y^2+h x y=0$
  • B
    $x^2-2 h x y+y^2=0$
  • Correct answer
    C
    $h x^2-h y^2+2 x y=0$
  • D
    $h x^2+h y^2-x y=0$

Solution

The bisectors of the angle between the lines in new position are same as the bisectors of the angles between their old positions. Therefore, the required equation is $$ \begin{aligned} \frac{x^2-y^2}{1-(-1)} & =\frac{x y}{-h} \Rightarrow h x^2-h y^2=-2 x y \\ \Rightarrow h x^2-h y^2+2 x y & =0 \end{aligned} $$

Hence, the correct answer is option: 'C'.

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