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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 the product of the other and a constant, the two are said to be ''directly proportional''. are directly proportional if the ratio is constant. *If the product of the two variables is always a constant, the two are said to be ''inversely proportional''. are inversely proportional if the product ''xy'' is constant. To express the statement "''y'' is (directly) proportional to ''x''" mathematically, we write an equation , for some real constant, ''c''. Symbolically, this is written . To express the statement "''y'' is inversely proportional to ''x''" mathematically, we write an equation . We can equivalently write "''y'' is proportional to 1/''x''", which would represent. If a linear function transforms into , and if the product is not zero, we say are proportional . An equality of two ratios such as , where no term is zero, is called a proportion. ==Direct proportionality== Given two variables ''x'' and ''y'', ''y is directly proportional to x'' (''x and y vary directly,'' or ''x and y are in direct variation'') if there is a non-zero constant ''k'' such that : The relation is often denoted, using the ∝ symbol, as : and the constant ratio : is called the proportionality constant, constant of variation or constant of proportionality. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Proportionality (mathematics)」の詳細全文を読む スポンサード リンク
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