三角函数 (4 19:19:25)

sin2x
=2sinx*cosx
=2sinx/cosx*cos^2 x
=2tanx/(1/cos^2x)
=2tanx/[(sin^2x+cos^2x)/cos^2x]
=2tanx/(1+tan^2x)
2tanx/（1+tan^2x） =3/5
sin2x=3/5
Cos2A=CosA^2-SinA^2=1-2SinA^2
sin^2(π/4 +x)
=[1-cos2(π/4 +x)]/2
=[1-cos(π/2 +2x)]/2
=[1+3/5]/2
=4/5