S=x^2+2xy+3y^2+2x+6y+4
S=(x+y)^2+2x+2y+1+2y^2+4y+3
S=(x+y)^2+2(x+y)+1+2(y+1)^2+1
S=(x+y+1)^2+2(y+1)^2+1>=1
当x+y+1=0
且y+1=0的时候,S取最小值1
即x=0,y=-1时,S最小为1