y=sin(2x+a)(0

y=sinx是奇函数,y=cosx是偶函数
a=π/2
cosx
=sin(π/2-x) 提一个负号,得
=-sin(x-π/2) 再加半个周期π,得
=sin(x-π/2+π)
=sin(x+π/2)