Apply the property of absolute values: if , then . By this property, is equivalent to . That's the same as saying that and.
Add to both sides of both inequalities:
and .
Divide both sides of both inequalities by . Note that because , dividing both sides of an equality by this number will flip the direction of the inequality sign.
would become .
would become .
Both inequalities are supposed to be true. Combining the two inequalities to obtain: