設f(sinx)=cos2x,則f(1/3)等於?

設f(sinx)=cos2x,則f(1/3)等於?

f(sinx)=cos2x=1-2(sinx)^2
所以f(x)=1-2x^2
f(1/3)=1-2*(1/3)^2=7/9