图像上相邻的两个最高点间的距离为2π=>T=2π
w=1
f(x)=sin(x+φ)
对称轴x+φ=kπ+(π/2)
φ=kπ+(π/2)
φ=π/2
f(x)=sin(x+(π/2))
=cosx
单调递减区间[2kπ,2kπ+π]