y=2cos(x+π/4)
x+π/4∈（2kπ-π,2kπ）时单调增
增区间x∈（2kπ-5π/4,2kπ-π/4）,其中k∈Z
y=sin(π/4-2x)=-sin(2x-π/4)
2x-π/4∈（2kπ-π/2,2kπ+π/2）时单调减
减区间x∈（kπ-π/8,kπ+3π/8）,其中k∈Z