Dice three rational numbers *a, b* and *ç*, non-null, is called **fourth proportional** of these numbers a number *x* such that:

Example:

- Determine the fourth proportional of the numbers 8, 12, and 6.
**Solution:**We indicate by**x**the fourth proportional and we set the proportion:

(applying the fundamental property)*8 x = 12. 6th**8 x = 72**x = 9*

So the fourth proportional is 9.