(-2x-2y)-[(x+y)²+3]
=-2(x+y)-(x+y)²-3
=-(x+y)²-2(x+y)-1-2
=-[(x+y)+1]²-2
=-(x+y+1)²-2
平方项恒非负,(x+y+1)²≥0
-(x+y+1)²≤0
-(x+y+1)²-2≤-2