若x+2的绝对值+(y-1)²=0,x+2 = 0 y-1 = 0 x= -2 y=1 则xy= -2 若绝对值a=2,则a= ±2 当a=2分之1,b=1时,代数式a²+3ab-b²的值= 1/4 + 3/2 - 1 = ( 1 + 6 - 4 ) / 4= 4分之 34(x²+xy-6)-3(2x²-xy)