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:
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