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Question

If the curves x to the power of 2 end exponent equals 9 A left parenthesis 9 minus y right parenthesis and x to the power of 2 end exponent equals A left parenthesis y plus 1 right parenthesis intersect orthogonally, then the value of A is

  1. 3  
  2. 4  
  3. 5  
  4. 7  

The correct answer is: 4


    On differentiating given curves respectively
    2 x equals negative 9 A fraction numerator d y over denominator d x end fraction blank rightwards double arrow blank open parentheses fraction numerator d y over denominator d x end fraction close parentheses subscript c subscript 1 end subscript end subscript equals negative fraction numerator 2 x over denominator 9 A end fraction

    And 2 x equals A blank fraction numerator d y over denominator d x end fraction

    rightwards double arrow blank open parentheses fraction numerator d y over denominator d x end fraction close parentheses subscript c subscript 2 end subscript end subscript equals fraction numerator 2 x over denominator A end fraction

    For orthogonally,

    open parentheses fraction numerator d y over denominator d x end fraction close parentheses subscript c subscript 1 end subscript end subscript open parentheses fraction numerator d y over denominator d x end fraction close parentheses subscript c subscript 2 end subscript end subscript equals negative 1

    rightwards double arrow blank open parentheses negative fraction numerator 2 x over denominator 9 A end fraction close parentheses open parentheses fraction numerator 2 x over denominator A end fraction close parentheses equals negative 1 ...(i)

    From given curves,

    9 A open parentheses 9 minus y close parentheses equals A open parentheses y plus 1 close parentheses blank rightwards double arrow blank y equals 8

    Since, x to the power of 2 end exponent equals 9 A open parentheses 9 minus y close parentheses

    rightwards double arrow blank x to the power of 2 end exponent equals 9 A open parentheses 9 minus 8 close parentheses equals 9 A

    From Eq. (i), fraction numerator 4 x to the power of 2 end exponent over denominator 9 A to the power of 2 end exponent end fraction equals 1

    rightwards double arrow blank fraction numerator 4.9 blank A over denominator 9 A to the power of 2 end exponent end fraction equals 1 blank rightwards double arrow blank A equals 4

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