y=sin^2x+2sinxcosx+3cos^2x=1+sin2x+1+cos2x=2+√2sin（2x+π/4）当2x+π/4=2kπ-π/2时，x=kπ-3π/8（k为整数）时，y有最小值2-√2集合{x│x=kπ-3π/8（k为整数）}
