sinx对称轴就是sinx=±1时,x的值
所以x=kπ+π/2
所以这里是2x+α=kπ+π/2
x=π/8
α=kπ+π/4
所以k=-1
α=-3π/4
f(x)=sin(2x-3π/4)
增函数时,2x-3π/4属于(2kπ-π/2,2kπ+π/2)
减函数是(2kπ-3π/2,2kπ-π/2)
2kπ-π/2