81(x+2y)^2-9(x-3y)^2=[9(x+2y)]^2-[3(x-3y)]^2————积的乘方=[9(x+2y)+3(x-3y)][9(x+2y)-3(x-3y)]————平方差公式=(9x+18y+3x-9y)(9x+18y-3x+9y)————去括号=(12x+9y)(6x+27y)————合并同类项=3(4x+3y)...