y=(sinx)^2+2sinxcosx+3(cosx)^2利用2倍角公式=(1-cos2x)/2+sin2x+3(1+cos2x)/2=cos2x+sin2x+2=√2sin(2x+π/4)+2由-1≤sin(2x+π/4)≤12-√2≤y≤2+√2取最小值时sin(2x+π/4)=-12x+π/4=2kπ+3π/2x=kπ+5π/8 k是...