若f(sin x)=3-cos 2x,则f(cos x)=( ) A.3-cos 2x B.3-sin 2x C.3+cos 2x D.3+sin 2x
问题描述:
若f(sin x)=3-cos 2x,则f(cos x)=( )
A. 3-cos 2x
B. 3-sin 2x
C. 3+cos 2x
D. 3+sin 2x
答
f(sinx)=3-cos2x=3-(1-2sin2x)=3-1+2sin2x=2sin2x+2,
则f(cosx)=2cos2x+2=2cos2x-1+3=3+cos2x,
故选:C.