cos(2α π/4)＝cos(2α)＊cos(π/4)－sin2α＊sin(π/4)
再把cos(α π/4)=3/5化简cos(α π/4)＝cos(α)＊cos(π/4)－sinα＊sin(π/4)＝3/5
根据这两个等式就可以得出.