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JEE Main20204 Sep 2020Morning ShiftMathematicsDifferentiationActual

If a + 2 b cos x ) ( a - 2 b cos y = a 2 - b 2 , where a > b > 0 , then d x d y at π 4 , π 4 is:

Options

  1. Aa - 2 b a + 2 b
  2. Ba - b a + b
  3. Ca + b a - b
  4. D2 a + b 2 a - b

Correct answer

C. a + b a - b

Step-by-step solution

a + 2 b cos x a − 2 b cos y = a 2 − b 2 a − 2 b cos y − 2 b sin x d x d y + 2 b sin y a + 2 b cos x = 0 d x d y = 2 b sin y a + 2 b cos x 2 b sin x a − 2 b cos y = sin y a + 2 b cos x sin x a − 2 b cos y d x d y π 4 , π 4 = a + b a − b .

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