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Let f : R → R be a twice differentiable function such that (sinx cosy)(f(2x + 2y) – f(2x – 2y)) = (cosx siny)(f(2x + 2y) + f(2x – 2y)), for all x, y ∈ R. If f ′ (0) = 2 1 , then the value of 24 f ′′ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ π 3 5 is :

1. 3
2. 2
3. –3
4. –2

Correct answer: –3

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