# Coordinate Geometry: Definition & Formulas

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• 0:38 Calculating Slope,…
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Lesson Transcript
Instructor: Mia Primas

Mia has taught math and science and has a Master's Degree in Secondary Teaching.

In this lesson we'll define coordinate geometry and learn about formulas that are commonly used in coordinate geometry. You can then take a brief quiz to see what you learned.

## Definition of Coordinate Geometry

There are all sorts of ways that we can find the measurements of lines and angles. We can use rulers to measure lines and protractors to measure angles. In coordinate geometry we can use graphs and coordinates to find measurements and other useful information about geometric figures.

Let's start by reviewing the features of coordinate graphs. A coordinate graph is a rectangular grid with two number lines called axes. The x-axis is the horizontal number line and the y-axis is the vertical number line. The axes intersect at the origin which is the point (0,0).

## Calculating Slope, Distance, and Midpoint

With coordinate geometry, various geometric figures can be graphed using the coordinates of the figure's vertices (corners). We can use these coordinates along with mathematical formulas to calculate the length, slope, and midpoint of the sides.

The subscripts of 1 and 2 in the formulas are used to distinguish the coordinates of each point. For example, x sub 2 is the x-coordinate for the second point and tells us where the point is along the x-axis. y sub 2 is the y-coordinate for the second point and tells us where the point is along the y-axis.

Check out this example:

Find the slope, length, and midpoint of side AB in triangle ABC.

To find the slope of side AB we'll substitute the coordinates of points A and B into the slope formula. It is helpful to label the coordinates with x sub 1, x sub 2, etc. to remember what values to substitute in the formula. The first value for each point is x, the second value is y. Then we add the subscripts of 1 and 2 for the first and second points. Once the coordinates are labeled, we can substitute them into the slope formula.

The slope of side AB is -5/3. The length of side AB is the same as the distance between points A and B, so we can use the distance formula.

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