sinx≥cosx
sinx-cosx≥0
√2(√2sinx/2-√2cosx/2)≥0
√2[sinxcos(-π/4)+sin(-π/4)cosx]≥0
√2sin(x-π/4)≥0
0≤x-π/4≤π
π/4≤x≤5π/4