Fourth, Mean and Third Proportional
The fourth proportional is any of the terms of a ratio.
A continued proportion has two equal means.
To calculate the proportional mean of a continued proportion, extract the square root from the product of the extremes.
In a continued proportion, the third proportional is the variable in a proportion with two equal terms.
A third proportional is equal to the square of the equal terms, divided by the unequal term.