4(x+y)^2+5-20(x+y-1)
=4(x+y)^2-20(x+y)+25
=(2x+2y)^2-10(2x+2y)+5^2
=(2x+2y-5)^2
2x和2y可不可以并成2(x+y)
最好写成(2x+2y-5)^2的形式