Maths-
General
Easy

Question

The common tangents to the circle x2 + y2 = a2/2 and the parabola y2 = 4ax intersect at the focus of the parabola-

  1. x2 = 4ay    
  2. x2 = –4ay    
  3. y2 = –4ax    
  4. y2 = 4a(x + a)    

The correct answer is: y2 = –4ax


    Equation of a tangent to the parabola y2 = 4ax is y = mx + a/m.
    fraction numerator a over denominator m end fraction equals open parentheses fraction numerator a over denominator square root of 2 end fraction close parentheses square root of 1 plus m to the power of 2 end exponent end rootrightwards double arrow 2 = m2 (1 + m2)
    rightwards double arrowm4 + m2 – 2 = 0 rightwards double arrow (m2 – 1) (m2 + 2) = 0
    rightwards double arrowm2 = 1 rightwards double arrow x = ± 1
    Hence the common tangents are y = x + a and y = –x –a which intersect at the point
    (–a, 0) which is the focus of the parabola y2 = –4ax.

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