The distance formula is derived from the pythagorean theorem. To find the distance between two points (x 1,y 1) and (x 2,y 2), all that you need to do is use the coordinates of these ordered pairs and apply the formula pictured below.
The distance formula is 
Below is a diagram of the distance formula applied to a picture of a line segment
Video Tutorial on the Distance Formula
Distance Formula is just the Pythagorean Theorem in Disguise
Interactive HTML5 Applet
(Applet on its own page)
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$$
\text{Distance }= \sqrt{ (x_2 - x_1)^2 - (y_2 - y_1)^2} $$
$$
$$
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Practice Problems
The Distance Formula
What is the distance between the two points on the left?
Now, let's find the distance. We're basically just using the pythagorean theorem.
Calculate the distance between the points (1,2) and (4,6).
Apply the distance formula to our points (1,2) and (4,6)
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Distance Formula Worksheet (pdf)
