In mathematics, two variables are proportional if a change in one is always accompanied by a change in the other, and if the changes are always related by use of a constant multiplier. The constant is called the coefficient of proportionality or proportionality constant. *If one variable is always [[wiktionary:product# the product]] of the other and a constant, the two are said to be ''directly proportional''. are directly proportional if the ratio \tfrac yx is constant.
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