Proportions

Proportion. Crosswise lied terms. Ratios. Overturned ratios. Derived proportions.
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.
E x a m p l e : If m = n = k = 1, l = 0, then we receive: a+b=c+d= a and c
a