f(x)=2cos²x -cos(2x+π/2)=1+cos(2x) +sin(2x)=1+√2 sin(2x+π/4)
f(π/8)=1+√2sin(π/2)=1+√2 Tmin=2π/2=π
2kπ+π/2≤2x+π/4≤2kπ+3π/2 kπ+π/8≤x≤kπ+5π/8 区间[kπ+π/8,kπ+5π/8]