Geometry For Dummies
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If two points in the x-y coordinate system are located diagonally from each other, you can use the distance formula to find the distance between them. As you will see, this distance is also the length of a hypotenuse.

Distance formula: To calculate diagonal distances, mathematicians whipped up the distance formula, which gives the distance between two points (x1, y1) and (x2, y2):

geometry-distance

Note: Like with the slope formula, it doesn't matter which point you call (x1, y1) and which you call (x2, y2).

This figure illustrates the distance formula.

geometry-hypoteneuse
 The distance between two points is also the length of the hypotenuse.

As the figure shows, the distance formula is simply the Pythagorean Theorem (a2 + b2 = c2) solved for the hypotenuse:

geometry-distance-formula

Take another look at the figure. The legs of the right triangle (a and b under the square root symbol) have lengths equal to (x2 – x1) and (y2 – y1). Remember this connection, and if you forget the distance formula, you'll be able to solve a distance problem with the Pythagorean Theorem instead.

Don't mix up the slope formula with the distance formula. You may have noticed that both formulas involve the expressions (x2 – x1) and (y2 – y1). That's because the lengths of the legs of the right triangle in the distance formula are the same as the rise and the run from the slope formula. To keep the formulas straight, just focus on the fact that slope is a ratio and distance is a hypotenuse.

About This Article

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About the book author:

Mark Ryan is the founder and owner of The Math Center in the Chicago area, where he provides tutoring in all math subjects as well as test preparation. Mark is the author of Calculus For Dummies, Calculus Workbook For Dummies, and Geometry Workbook For Dummies.

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