（x-y）²>=0,√(5x-3y-6)>=0,
互为相反数,（x-y）²=0,x-y=0,x=y;
√(5x-3y-6)=0,5x-3y-6=0,5x-3x-6=0,2x=6x=3=y,
√(x²+2xy+y²)=√(x+y)²=√(3+3)²=6.