cos2x = cos^2x-sin^2x的推导

cos^2x-sin^2x=cos2x的推导

利用两角和的余弦公式cos2X = cos(X+X) = cosXcosX- sinXsinX = cos^2 X- sin^2 X = 2cos^2 X- 1=1 - 2sin^2 X

两角和与差的余弦公式的五种推导方法之对比

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