properties of inequalities
1. Any real number can be added to (or subtracted from) both sides of an inequality, producing a new inequality with the same direction as the original one.
2. Both sides of an inequality can be multiplied (or divided) by a positive number to produce a new inequality with the same direction as the original one.
3. Both sides of an inequality can be multiplied (or divided) by a negative number to produce a new inequality that has the opposite direction of the original one.