Proportion is an equality of two ratios. From a proportion follows:
ad = bc (cross products are equal). Inversely, from ad = bc the next proportions follow: All these proportions and some others can be received from the original proportion
a / b = c / d by the next rules:

Crosswise lied terms of any proportion can be changed by places.

Ratios in any proportion can be changed to overturned ones.

Derived proportions. If then the next so called derived proportions, received from the original, also take place: All these and a lot of other proportions can be united by the two base formulas: where m, n, k, l – any numbers.

Example: If m = n = k = 1, l = 0, then we receive: