Equations

RULE 19

By this method of reasoning, as many magnitudes expressed in two different ways must be sought as the number of unknown terms we assume to be known in order to proceed directly through the difficulty; for thus, there will be as many comparisons between equal things.

RULE 20

Once the equations have been found, the operations that were omitted must be completed, never using multiplication when division is possible.

RULE 21

If there are several such equations, all of them must be reduced to a single one—namely, the one whose terms occupy the fewest degrees in the series of continuously proportional magnitudes, according to which the same are to be arranged in order.