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.


  • 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.

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