# Third Proportional

## Third Proportional

If a, b and c are numbers connected by the proportion $$\dfrac{a}{b}$$ = $$\dfrac{b}{c}$$, then c = $$\dfrac{b^{2}}{a}$$ and c is called the third proportional.

In a proportion with three terms that are, in order, ab and c, the third proportional is c.

### Examples

• In the proportion $$\dfrac{x}{12}$$ = $$\dfrac{12}{16}$$, 9 is the third proportional, because x = 9.
• In the proportion $$\dfrac{12}{18}$$ = $$\dfrac{18}{x}$$, 27 is the third proportional, because x = 27.