Let sinx = t, f (t) = 2-2t 2, f (cosx) = 2-2cos2x = 2 (1-cos2) = 2sin2x.

2.f(x+ 1/x)=x^2+ 1/x^2=(x+ 1/x)^2-2

Let x+ 1/x=t, (t≤-2 or t≥2)

F (t) = t 2-2, that is, f (x) = x 2-2 (x ≤-2 or x≥2).