In mathematics, two quantities are proportional if they vary in such a way that one of them is a constant multiple of the other.
Contents
Symbol
The mathematical symbol '∝' is used to indicate that two values are proportional. For example, A ∝ B.
In Unicode this is symbol U+221D.
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 nonzero constant k such that
The relation is often denoted
and the constant ratio
is called the proportionality constant or constant of proportionality.
Examples
 If an object travels at a constant speed, then the distance traveled is proportional to the time spent traveling, with the speed being the constant of proportionality.
 On a map drawn to scale, the distance between any two points on the map is proportional to the distance between the two locations that the points represent, with the constant of proportionality being the scale of the map.
Properties
Since
is equivalent to
it follows that if y is proportional to x, with (nonzero) proportionality constant k, then x is also proportional to y with proportionality constant 1/k.
If y is proportional to x, then the graph of y as a function of x will be a straight line passing through the origin with the slope of the line equal to the constant of proportionality: it corresponds to linear growth.
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