整式的乘除：已知a+b＝5,ab=21／4,求值：|a－b|

题目

整式的乘除：已知a+b＝5,ab=21／4,求值：|a－b|

答案

a+b＝5,ab=21／4,
(a－b)²
=(a+b)²-4ab
=5²-21
=4
|a-b|=2