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Tool: Law of Cosines - Math Open Reference
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A web page and interactive applet demonstrating the Law of Cosines.

The applet shows a triangle with draggable vertices. As you drag them, the Law of Cosines formula is continually recalculated, showing that you can find the third side of any triangle if you are given two sides and the included angle.

The text on the web page explains the concept and has a worked example. It suggests an activity where the user turns off the onscreen calculation, tries it themselves and then turns the calculation back on to check their answer.

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at

Technology Type: Flash 
Author: John Page
Language: English 
Cost: Does not require payment for use

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Courses: Geometry Law of cosines