x+y-2=0x+y=2（x²-y²）²-8（x²+y²）=(x+y)²﹙x-y)²-8x²-8y²=4(x²-2xy+y²)-8x²-8y²=4x²-8xy-4y²-8x²-8y²=-4(x²+2xy+y&#...=(x+y)²﹙x-y)²-8x²-8y²=4(x²-2xy+y²)-8x²-8y²请问这两步是怎么做的(x+y)²﹙x-y)²-8x²-8y² x+y=2代入（x-y)²=x²-2xy+y²=4(x²-2xy+y²)-8x²-8y²=4(x²-2xy+y²)-8x²-8y²=4x²-8xy-4y²-8x²-8y²好像哪写错了4(x²-2xy+y²)-8x²-8y²=4x²-8xy+4y²-8x²-8y²