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令sinx=u,則(cosx)^2=(1-u^2),f(u)=sin3x=sin(2x+x)=sin2xcosx+cos2xsinx=2sinx(cosx)^2+(1-2(sinx)^2)sinx=2u(1-u^2)+(1-2u^2)u=3u-4u^3,則f(cosx)=3cosx-4(cosx)^3.
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