## Precalculus: Mathematics for Calculus, 7th Edition

$|a-b|=b-a$
$|a-b|,$ where $a\lt b$ The problem specifies that $a\lt b$. This means that the substraction inside the absolute value bars will result in a negative number. To express this quantity without using absolute value, we must change its sign: $|a-b|=-(a-b)=b-a$