In proportion we have two equal ratios. The equality is expressed by the sign:: which may be substituted by the international sign of equality =.
a: b:: c: d or a: b = c: d – is read: a is to b as c is to d;
2: 3:: 4: 6 or 2: 3 = 4: 6 – is read: two is to three as four is to six.
The extreme terms of proportion are called “extremes”, the mean terms are called “means”. The proportion can vary directly (изменяться прямо пропорционально) and it can vary inversely (изменяться обратно пропорционально):
x (y: x varies directly as y; x is directly proportional to y;
x = k/y: x varies inversely as y; x is inversely proportional to y.