# How do you factor by grouping: 1-a+ab-b?

Mar 4, 2018

Group 'em together
$a b - a + 1 - b$

$a b - a = a \left(b - 1\right)$
Notice that there will be a 1 as without it it'll simply be $a b$

$1 - b = 1 \left(1 - b\right)$
Notice that it doesn't match with the upper one... so we'll change the signs
$1 \left(1 - b\right) = - 1 \left(b - 1\right)$(try to multiply them now!!

Jot them down in one expression
$a \left(b - 1\right) - 1 \left(b - 1\right)$
You get!!!!!!
$\left(a - 1\right) \left(b - 1\right)$