Rule

Addition Property of Inequality

Adding the same number to both sides of an inequality generates an equivalent inequality. This equivalent inequality will have the same solution set and the inequality sign remains the same. Let x, y, and z be real numbers such that x< y. Then, the following conditional statement holds true.

If x< y, then x+z< y+z.

This property holds for the other types of inequalities.

The Addition Property of Inequality for All Types of Inequalities

Proof

Addition Property of Inequality
The case when x< y will be proven. Before starting the proof, consider the following biconditional statement. x< y ⇔ y-x> 0 Now the Identity Property of Addition can be applied to the second part of the statement.

y-x> 0
y-x+0> 0

Rewrite 0 as z-z

y-x+z-z> 0
y+z-x-z> 0

- a-b=-(a+b)

y+z-(x+z)> 0
(y+z)-(x+z)> 0

The last inequality can be rewritten using the biconditional statement. (y+z)-(x+z)> 0 ⇕ x+z< y+z Finally, because x< y, the property is obtained.

If x< y, then x+z< y+z.

Exercises
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