Question

若f(cosx)=3-cos2x,则f(sinx)=?

Answer 1

∵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