Two quantities are said to be proportional if their ratio is constant. In other words, the two quantities are proportional if one of them is always the product of the other and a constant quantity called coefficient of proportionality or proportionality constant.
The symbol 
is used to show that two quantities are proportional.
Direct Proportionality
The direct proportionality relation is denoted as
Two quantities are x and y are directly proportional (or vary directly or in direct variation)if
y = kx
where k is constant of proportionality (or proportionality constant), given as
k = y/x
Also,
x = (1/k)y
If y is proportional to x, with proportionality constant k, then x is also proportional to y with proportionality constant 1/k.
Example: Circumference of a circle is directly proportional to its diameter. Circle with larger diameter has greater circumference.
Inverse Proportionality
Two quantities are inversely proportional (or inverse proportion, or in inverse variation) if their product is a constant or if one of the variables is directly proportional to multiplicative inverse of the other.
The inverse proportionality is denoted as
y = k/x
k = xy
Example: Time to cover distance is inversely proportional to speed. Faster is the speed lesser is the time taken to travel the same distance.
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