x^2 - (a+b)x + ab = (x-a)(x-b) = 0 所以其2根分别是a 和 b 若方程：x^2 - abx + (a+b) = 0 有1根x = a,代入,得：a^2 - a^2b + a + b = 0 (b-1)a^2 - a - b = 0 ( (b-1)a - b ) ( a + 1 ) = 0 得：a = b/(b-1) ,或 ...