f(x)=sin(x+5π/4)+cos(x+3π/4)
=2cos(x+3π/4)
x+3π/4,[2Kπ,2Kπ+π]递减,x,[2Kπ-3π/4,2Kπ+π/4]递减
f(a)=2√2/3,对不?