Maths-
General
Easy

Question

PSQ is a focal chord of a parabola whose focus is S & vertex A . PA and QA are produced to meet the directrix in R and T respectively then straight angle RST equals

  1. 90º    
  2. 60º    
  3. 45º    
  4. 30º    

The correct answer is: 90º


    To find angle straight angle RST equals
    Let parabola y squared equals 4 a x
    Equation of TAQ = y = open parentheses fraction numerator begin display style fraction numerator negative 2 a over denominator t end fraction end style over denominator begin display style a over t squared end style end fraction close parentheses x
    y=(-2t)x
    At x=-a;
    y=2at
    T=(a,2at)
    Equation of RAP is y=open parentheses open parentheses fraction numerator 2 a t over denominator a t squared end fraction close parentheses close parentheses x
    R equals open parentheses negative a comma fraction numerator negative 2 a over denominator t end fraction close parentheses

S l o p e space o f space S T equals m 1 equals negative t

S l o p e space o f space S R equals m 2 equals 1 over t

tan left parenthesis R S T right parenthesis equals fraction numerator m 2 minus m 1 over denominator 1 plus m 1 m 2 end fraction equals 1 over 0 equals infinity

angle R S T equals 90 degree

    Therefor angle angle R S T equals 90 degree

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