Tính đạo Hàm Của Hàm Số F(x)=sin4x+cos4x Tại điểm X=π8

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Tính đạo hàm của hàm số fx=sin4x+cos4x tại điểm x=π8 . A. f′π8=34. B. f′π8=1. C. f′π8=−1. D. f′π8=0. Đáp án và lời giải Đáp án:C Lời giải:Lời giải Ta có fx=sin2x+cos2x2−2sin2xcos2x=1−12sin22x=34+14cos4x →f′x=−sin4x. Suy ra f′π8=−sin4. π8=−sinπ2=−1. Chọn C

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