Rule

Division Property of Inequality

Dividing both sides of an inequality by a nonzero real number z produces an equivalent inequality. However, the following conditions need to be considered.

Positive z If z is positive, the inequality sign remains the same.
Negative z If z is negative, the inequality sign needs to be reversed to produce an equivalent inequality.

For example, let x, y, and z be real numbers such that x

  • If x0, then xz< yz.
  • If x yz.

This property holds for the other types of inequalities.

The Division Property of Inequality for All the Types of Inequalities

Proof

Division Property of Inequality
The case when x

  • xif and only if y-x is positive.
  • If x and z are positive, then xz is also positive.
  • If z is negative, then - z is positive.

The following conditional statements can be proven using these properties.

  • If x0, then xz< yz.
  • If x yz.

Each case will be analyzed separately.

z>0

As it is given that xgreater than 0. x0 Furthermore, because z>0, from the second property, it can be stated that y-x divided by z is also greater than 0. y-x>0 &and z>0 &⇓ y-x/z&>0 Now the second part of this conditional statement can be rewritten. y-x/z>0 ⇔ y/z-x/z>0 By the first property, it can be said that xz is less than yz. Then because because x

If x0, then xz< yz.

z<0

The following statement is valid because x0 Additionally, since z<0, it follows that - z is positive by the third property. Moreover, the quotient of y-x and - z will be positive. y-x>0 &and - z>0 &⇓ y-x/- z&>0 Now the second part of this statement can be rewritten.

y-x/- z>0
Simplify
-(y-x)/z>0
x-y/z>0
x/z-y/z>0
x/z>y/z

Finally, the property has been obtained because x

If x yz.

Exercises
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