Third Proportional
If a, b and c are numbers connected by the proportion [latex]\dfrac{a}{b}[/latex] = [latex]\dfrac{b}{c}[/latex], then c = [latex]\dfrac{b^{2}}{a}[/latex] and c is called the third proportional.
In a proportion with three terms that are, in order, a, b and c, the third proportional is c.
Examples
- In the proportion [latex]\dfrac{x}{12}[/latex] = [latex]\dfrac{12}{16}[/latex], 9 is the third proportional, because x = 9.
- In the proportion [latex]\dfrac{12}{18}[/latex] = [latex]\dfrac{18}{x}[/latex], 27 is the third proportional, because x = 27.