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(4) 51. In every Geometrical proportion the product of the extreem is equal to the product of the mean.

Thus if a:b::c:d, is unclear For by the supposed unclear formula

52. This property of proportional numbers is the foundation of the rule of three in arithmetick. For if a:b::c:d. Then ad = cd. And dividing by a we have d = bc/a.

That is if four numbers by proportional & the three first be known, the fourth may be found by multiplying the second & third together & dividing by the first; which is the common arithmetical rule.

53. The converse of the foregoing proposition is also true, that four quantities are proportional if the product of the extreems is equal to the