x（x - 1）/ 2 = 45x（x - 1）= 45 × 2 x ² - x = 90x ² - x - 90 = 0（x - 10）（x + 9）= 0 x - 10 = 0 x + 9 = 0 x1 = 10x2 = - 9【俊狼猎英...