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Let f k (x) = k 1 (sin k x + cos k x) for k = 1, 2, 3, .... Then for all x ∈ R, the value of f 4 (x) – f 6 (x) is equal to

1. 4 1
2. 12 5
3. 12 1 −
4. 12 1

Correct answer: 12 1

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