Maths-
General
Easy

Question

If the line 2x – 1 = 0 is the derectrix of the parabola y2 – kx + 6 = 0 then one of the value of K is -

  1. – 6    
  2. 6    
  3. fraction numerator 1 over denominator 4 end fraction    
  4. fraction numerator 1 over denominator 4 end fraction    

The correct answer is: – 6


    The parabola is y2 = k open parentheses x – fraction numerator 6 over denominator K end fraction close parentheses &
    Dirctrix x – fraction numerator 6 over denominator K end fraction = – fraction numerator K over denominator 4 end fractionrightwards double arrowx = fraction numerator 6 over denominator K end fraction – fraction numerator K over denominator 4 end fraction
    Which coincides with line x = ½
    rightwards double arrowfraction numerator 6 over denominator K end fractionfraction numerator K over denominator 4 end fraction = fraction numerator 1 over denominator 2 end fractionrightwards double arrow K2 + 2K – 24 = 0
    rightwards double arrow (K + 6) (K – 4) = 0 rightwards double arrow K = – 6, K = 4

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