cos(π/4)=1-2sin²(π/8)
2sin²(π/8)=1-cos(π/4)
sin(π/8)=√{[1-cos(π/4)]/2}
=√[(1-√2/2)/2]
=√[(2-√2)/4]
=[√(2-√2)]/2
sin π/8=SQR(sin^2 π/8)=SQR[1/2-（1/2)cosπ/4]=?下来你自己算吧？记住公式：sin^2(A)=1/2(1-cos2A)
