(1) 因为x1+x2 = 1/3 x1*x2 = -1 所以(x1-x2)^2 = (x1+x2)^2 - 4x1*x2 = 1/9 + 4 = 37/9
所以|x1-x2| = (根号37) / 3;
(2)x1^3 = x1^2 * x1 = (x1+3)/3 * x1 = (x1^2 + 3x1)/3 = ((x1+3)/3 + 3x1)/3 = 10/9 * x1 + 1/3
同理x2^3 = 10/9 * x2 + 1/3 所以x1^3+x2^3 = 10/9 * (x1+x2) + 2/3 = 10/9 * 1/3 + 2/3 = 28/27
这里利用方程降次的观念需要注意 不用去死记立方和公式 (不断利用x^2 = (x+3)/3降次)