x²+3x+y-3=0, y=-x^2-3x+3,x+y=-x^2-2x+3,x=-1时,x+y的最大值为6
x+y=-x²-2*x+3=-(x²+2*x-3)=-((x+1)^2-4)=4-(x+1)^2所以x+y最大值为4，此时x=-1
