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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.

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