# Proof of angle addition formula for cosine | Trigonometry | Khan Academy | Summary and Q&A

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April 2, 2014
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Proof of angle addition formula for cosine | Trigonometry | Khan Academy

## TL;DR

This video provides a step-by-step proof of the angle addition formula for cosine, demonstrating that cosine of X plus Y is equal to cosine X times cosine Y minus sine X times sine Y.

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### Q: How is cosine of X plus Y expressed in terms of segments in the video?

Cosine of X plus Y is represented as the length of segment AF, which is equivalent to segment AB minus segment FB.

### Q: How does the proof establish that segment AB is equal to cosine X times cosine Y?

By considering right triangle ACB, it is shown that segment AB, adjacent to angle Y, is equal to cosine of X times cosine of Y.

### Q: How is segment FB defined in the proof?

Since ECBF is a rectangle, segment FB is equivalent to segment EC. By applying sine of Y, it is proven that segment FB is equal to sine X times sine Y.

### Q: What is the final expression for cosine of X plus Y in the proof?

Cosine of X plus Y is equal to cosine X times cosine Y minus sine X times sine Y.

## Summary & Key Takeaways

• The video aims to prove the angle addition formula for cosine.

• It uses triangles and trigonometric ratios to express cosine of X plus Y as the difference of two segments.

• The proof shows that segment AB is equal to cosine X times cosine Y, and segment FB is equal to sine X times sine Y.

• By subtracting FB from AB, the video concludes that cosine of X plus Y is equal to cosine X times cosine Y minus sine X times sine Y.