# proportion

Proportion is a statement that two ratios are equivalent,
written as *a*:*b* = *c*:*d*, as in the statement
2:3 = 6:9. Two functions are proportional
if for all *x*, *f*(*x*) = *kg*(*x*),
where *k* is the constant of proportionality.
If the value of *k* is not known, the statement may be written *f*(*x*) *g*(*x*). A **continued proportion** is a group of three or more quantities, each bearing the same ratio to its successor, such as the sequence 1:3:9:27:81.