Fourth term in a proportion with four terms.

In the proportion with four terms \(\dfrac {a}{b} =\dfrac {c} {d}\), the term

*d*is the fourth proportional.### Example

\(\dfrac {5}{8} =\dfrac {25} {x}\)

By calculating, we find that the 4th proportional, which is the value of the unknown *x*, is 40.