△若x^2+2xy+y^2-a(x+y)+25是完全平方式若x^2+2xy+y^2-a(x+y)+25是完全平方式,求a的值.以前的东西都忘了,给讲讲啊~

(x+y)^2-a(x+y)+25
{(x+y)-5}^2=（x+y)^2-10(x+y)+25
那a=2*5=10

(x+y+m)^2=x^2+2xy+y^2+2m(x+y)+m^2=x^2+2xy+y^2-a(x+y)+25
解得m=-5或5
a=10或-10

原式=(x+y)²-a(x+y)+25
=(x+y+5)²或(x+y-5)²
(x+y+5)²
=(x+y)+10(x+y)+25
-a=10
(x+y-5)²
=(x+y)²-10(x+y)+25
-a=-10
所以a=-10,a=10

[(x+y)+5]^2
a=-10