∵f(sinx)=3-cos2x=3-(1-2sinx*sinx)
∴f(x)=3-(1-2x*x)
=2+2x*x
或者:设sinx=t
cos2x=1-2(sinx)^2=1-2t^2
f(sinx)=3-cos2x
f(t)=3-(1-2t^2)=2t^2+2
把t改成x
f(x)=2x^2+2
f(cosx)=2cos^2 x+2=1+cos2x+2=3+cos2x
或者f(sinX)=3-cos2X
=3-[1-2(sinx)^2]
=2+2(sinx)^2
所以 f(x)=2+2x^2
所以f(cosx)=2+2(cosx)^2
=3+[2(cosx)^2-1]
=3+cos2x
参考资料:http://zhidao.baidu.com/question/33560508.html