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If variables x and y depend on each other via formula y = a / x (a ≠ 0} then they are inversely proportionality. Domain: R \ {0}. Image: R \ {0}. Examples: Time t needed to drive from town to town is inversely proportional to velocity v: t = s / v. Number of gallons of gas n is inversely proportional to price c per gallon, if refuel every time for the same money s: n = s / c. Graph of inverse proportionality y = a / x is hyperbola.