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Let f(x) = R x , x cos 4 b x sin 4 a x 2 2        be a function which satisfies      2 / 0 dy ) y ( f ) y x sin( x ) x ( f . Then ( a + b ) is equal to

1. –  (  + 2)
2. –  (  – 2)
3. – 2  (  + 2)
4. – 2  (  – 2)

Correct answer: – 2  (  + 2)

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