# Direct Variation

Pronunciation: /dɪˈrɛkt ˌvɛər.iˈeɪ.ʃən/ Explain

 Click on the point on the blue slider and drag it to change the figure. Which value of the constant of variation would indicate a gas price of \$3.00 per gallon? Manipulative 1 - Direct Variation Created with GeoGebra.

Direct variation means the output varies from the input by a ratio.[3] 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 y-intercept of zero. The graph of the direct variation above looks likes this:

### References

1. McAdams, David E.. All Math Words Dictionary, direct variation. 2nd Classroom edition 20150108-4799968. pg 61. Life is a Story Problem LLC. January 8, 2015. Buy the book
2. direct proportion. merriam-webster.com. Encyclopedia Britannica. Merriam-Webster. Last Accessed 7/3/2018. http://www.merriam-webster.com/dictionary/direct proportion. Buy the book
3. Bettinger, Alvin K. and Englund, John A.. Algebra and Trigonometry. pp 57-59. www.archive.org. International Textbook Company. January 1963. Last Accessed 7/3/2018. http://www.archive.org/stream/algebraandtrigon033520mbp#page/n74/mode/1up/search/variation. Buy the book
4. Young, J. W. A. and Jackson, Lambert L.. A First Course in Elementary Algebra. pp 167-174. www.archive.org. D. Appleton and Company. 1910. Last Accessed 7/3/2018. http://www.archive.org/stream/firstcourseinele00younrich#page/167/mode/1up/search/direct+variation. Buy the book

• McAdams, David E.. Directly Proportional. allmathwords.org. All Math Words Encyclopedia. Life is a Story Problem LLC. 1/23/2010. https://www.allmathwords.org/en/d/directlyproportional.html.

McAdams, David E. Direct Variation. 12/21/2018. All Math Words Encyclopedia. Life is a Story Problem LLC. https://www.allmathwords.org/en/d/directvariation.html.

### Revision History

12/21/2018: Reviewed and corrected IPA pronunication. (McAdams, David E.)