(3a+b)(_b-3a__)=b²-9a²
长方形周长为20 两条邻长x y,满足2(x-y)-x²+2xy-y²-1=0
2(x-y)-x²+2xy-y²-1=0,则2(x-y)-(x-y)²-1=0
即-（(x-y)-1）²=0
设x为长.y为宽
x+y=10,x-y=1
x=5.5,y=4.5