
Direct variation means the output varies from the input by a ratio.^{[1]} The general form of the equation for direct variation y = mx where m is the constant of variation. An example of direct variation is buying gas. When you pump the gas, the total price varies directly as a function of the number of gallons of gas you pump. The constant of variation is the price per gallon. So, if gas costs $2.50 per gallon, the equation is y = $2.50x where x is the number of gallons pumped and y is the total price. If you pump 10 gallons of gas, the price is $25.00. Note that direct variation always has a yintercept of zero. The graph of the direct variation
above looks likes this:

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