2(x-y)平方+20(x-y)+50

2(x-y)平方+20(x-y)+50
=2((x-y)平方+10(x-y)+25)
=2(x-y+5)平方

2(x-y)^2+20(x-y)+50 = 0
约掉公因数2，即(x-y)^2+10(x-y)+25 = 0
剩下的部分刚好是完全平方的形式，
（x-y+5）^2=0
解得x-y=-5

原式=2(x-y)^2+20(x-y)+5*10.....将50分解
=(x-y+5)(2(x-y)+10)............因式分解
=2(x-y+5)^2....................提取2

2(x-y+5)^2