f(cosx)=cos2x,则f(sin30°)=

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f(cosx)=cos2x,则f(sin30°)=
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f(cosx)=cos2x,则f(sin30°)=
f(cosx)=cos2x,则f(sin30°)=

f(cosx)=cos2x,则f(sin30°)=
f(cosx)=2Cos^2x-1
f(x)=2x^2-1
f(sin30)=f(1/2)=2*1/4-1=-1/2

cos2x=2(cosx)^2-1
f(sin30°)=2sin30°^2-1
=-1/2

sin30°=1/2
f(1/2)=(1/2)^2-(√3/2)^2=-1/2