f(x)=sin²x+2sinxcosx+3cos²x=sin2x +2cos²x +1=sin2x+cos2x +2=√2[(√2/2)sin2x+(√2/2)cos2x] +2=√2sin(2x+π/4) +2（1）令 -π/2+2kπ≤2x+π/4≤π/2+2kπ解得 -3π/8+kπ≤x≤π/8+kπ从而 增...