原式 (5x+m)^2 = 25x^2-10xy+n
左边平方得 25x^2+10mx+m^2 = 25x^2-10xy+n
根据等式两边各项应该对应相等
则有+10mx = -10xy
+m^2 = n
∴m = -y n = m^2 = y^2