x^2+2xy+y^2-a(x+y)+25 = (x+y)^2 - a(x+y) + 5^2
-a = +-(2*5)
即a = 10 或 -10第二行不懂，怎么解的(m+n)^2 = m^2 + 2*m*n + n^2 (m-n)^2 = m^2 - 2*m*n + n^2 把m看做(x+y),n看做5