等差中项:2cosx=sinb+cosb ------(1)
等比中项:2(sinx)^2=sinb*cosb ---(2)
(1)式两边平方有:4(cosx)^2=(sinb)^2+(cosb)^2+2sinbcosb=1+2sinbcosb
将(2)代入,即
4(cosx)^2=1+4(sinx)^2
(cosx)^x-(sinx)^2=1/4
所以
cos2x=(cosx)^2-(sinx)^2
=1/4
等差中项:2cosx=sinb+cosb ------(1)
等比中项:2(sinx)^2=sinb*cosb ---(2)
(1)式两边平方有:4(cosx)^2=(sinb)^2+(cosb)^2+2sinbcosb=1+2sinbcosb
将(2)代入,即
4(cosx)^2=1+4(sinx)^2
(cosx)^x-(sinx)^2=1/4
所以
cos2x=(cosx)^2-(sinx)^2
=1/4