Maths-
General
Easy

Question

If the chord of contact of tangents from a point P to the parabola y2 = 4ax, touches the parabola x2 = 4by, then the locus of P is a/an -

  1. Circle    
  2. Parabola    
  3. Ellipse    
  4. Hyperbola    

The correct answer is: Hyperbola


    Let P(h, k) be a point. Then the chord of contact of tangents from P to y2 = 4ax is
    ky = 2a (x + h)
    This touches the parabola x2 = 4by. So, it should be of the form
    x = my + fraction numerator b over denominator m end fraction… (2)
    Equation (1) can be re-written as
    x = open parentheses fraction numerator k over denominator 2 a end fraction close parenthesesy – h… (3)
    Since Equation (2) and (3) represent the same line.
    thereforem = fraction numerator k over denominator 2 a end fraction and fraction numerator b over denominator m end fraction = –h
    Eliminating m from these two equations, we get
    2ab = –hk
    Hence, the locus of P(h, k) is
    xy = –2ab, which is a hyperbola.
    Hence (D) is the correct answer.

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