f(x)=sin2x+cos2x.f(x) √2sin(2x+π/4).f(α/2+π/8)=√2sin[2(α/2+/8+)π/4]=3√2/5.sin(α+π/4+π/4)=3/5.sin(α+π/2)=3/5.cosα=3/5.cos2α＝2ccos^2α-1.=2*(3/5)^2-1.∴cos2α=-7/25.