题目

己知a-b=5,ab=-1,求a和b的值
则3a-3(ab+b)的值是？

提问时间：2022-01-05

答案

a - b = 5
ab = - 1
b = a - 5
a(a - 5) = - 1
a² - 5a + 1 = 0
a² - 5a + 25/4 = 21/4
(a - 5/2)² = 21/4
a - 5/2 = ± √21 / 2
a = 5/2 ± √21 / 2
b = a - 5 = - 5/2 ± √21 / 2
3a-3(ab+b)
= 3(a - b) - 3ab
= 15 + 3
= 18