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
https://wenku.baidu.com/view/63fdb27ba31614791711cc7931b765ce04087a7d.html
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
https://wenku.baidu.com/view/63fdb27ba31614791711cc7931b765ce04087a7d.html