Question

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

Answer 1

Group 'em together
`ab-a+1-b`

`ab-a=a(b-1)`
Notice that there will be a 1 as without it it'll simply be `ab`

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

Jot them down in one expression
`a(b-1)-1(b-1)`
You get!!!!!!
`(a-1)(b-1)`