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General
Easy

Question

AB, AC are tangents to a parabola y2 = 4ax. If l1, l2, l3 are the lengths of perpendiculars from A, B, C on any tangent to the parabola, then -

  1. l1, l2, l3 are in G.P.    
  2. l2, l1, l3 are in G.P.    
  3. l1, l3, l2 are in G.P.    
  4. l3, l2, l1 are in G.P.    

The correct answer is: l2, l1, l3 are in G.P.


    Let the coordinates of B and C be (at12, 2at1) and (at22, 2at2) respectively. Then, the coordinates of A are (at­1t2 , a(t1 + t2)).
    The equation of any tangent to y2 = 4ax is
    ty = x + at2
    thereforel1 = fraction numerator a t subscript 1 end subscript t subscript 2 end subscript minus a left parenthesis t subscript 1 end subscript plus t subscript 2 end subscript right parenthesis t plus a t to the power of 2 end exponent over denominator square root of 1 plus t to the power of 2 end exponent end root end fraction,
    l2 = fraction numerator a t subscript 1 end subscript superscript 2 end superscript minus 2 a t t subscript 1 end subscript plus a t to the power of 2 end exponent over denominator square root of 1 plus t to the power of 2 end exponent end root end fraction
    andl3 = fraction numerator a t subscript 1 end subscript superscript 2 end superscript minus 2 a t t subscript 2 end subscript plus a t to the power of 2 end exponent over denominator square root of 1 plus t to the power of 2 end exponent end root end fraction
    Clearly, l 2 l 3 = l12. Therefore, l 2, l 1, l 3 are in G.P.
    Hence (B) is the correct answer.

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