2sinπ/8cosπ/8怎么得到sinπ/4?

2sinπ/8cosπ/8怎么得到sinπ/4? 为什么3sin^2(3π/2-x)=3cos^2x 为什么sin(3π/2)cosx-cos(3π/2)sinx?sin(3π/2)cosx-cos(3π/2)sinx = -cosx 是怎么得到的？ sin cos之间怎么转换?sin(π/3-2X）等于cos多少? sinπ/8×cosπ/8 sinπ/8×cosπ/8 sinπ/8*cosπ/8 sinπ/8cosπ/8=( ) 化简sin(α-5π)/cos(3π-α)×cos(π/2-α)/sin(α-3π)×cos(8π-α)/sin(-α-4π) f(x）=cos⁴x+2sinxcosx-sin⁴x =(cos⁴xx-sin⁴x) +2sinxcos=(cos²x+sin²x)(cos²x-sin²x)+2sinxcosx=cos2x+sin2x=√2sin(2x+π/4)那个cos2x怎么得到的 用了什么公式还是什么定理 谢啦. cos^6(π/8)-sin^6(π/8)=求值,[cos^2(π/8)-sin^2(π/8)][cos^4(π/8)+sin^4(π/8)+cos^2(π/8)sin^2(π/8)]=cos(π/4)[1-cos^2(π/8)sin^2(π/8)]=cos(π/4)[1-[sin^2(π/4)]/4]==7√2/16 √2sin(x/2+π/4)cos(x/2)=√2/2sin(x+π/4)+ 1/2 怎么得到的 tan（θ+α）=cosθ（1-sinθ）,怎么得到sin(π/4-θ/2-a)=0 sin（a+b）的推导过程sin(a+b)=cos[π/2-(a+b)]=sinacosb+cosasinb 怎么得到的 麻烦写具体一点吧 (cosπ/8+sinπ/8)（cos^3π/8-sin^3π/8） (cosπ/8+sinπ/8)（cos^3π/8-sin^3π/8）. (cos π/8+ sin π/8 ) (cos^3 π/8+sin^3 π/8 )的值. cos 5π/8 cos π/8 + sin 5π/8 sin π/8= 化简：[1+2sin(α+2π)*cos(α-2π)]/[sin(α+4π)+cos(α+8π)]