tan(a+π/4)
=(tana+tanπ/4)/(1-tanatanπ/4)
=(tana+1)/(1-tana)
=k
∴tana=(k-1)/(k+1)
cos2a
=cos²a-sin²a
=(cos²a-sin²a)/1
=(cos²a-sin²a)/(cos²a+sin²a)
分子分母同除cos²a得
=(1-tan²a)/(1+tan²a)
=(2k)/(k²+1)
手机提问的朋友在客户端右上角评价点【满意】即可