# Introduction to the unit circle | Trigonometry | Khan Academy | Summary and Q&A

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November 19, 2012
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Introduction to the unit circle | Trigonometry | Khan Academy

## TL;DR

This video explains how to use the unit circle to extend our traditional definitions of trigonometric functions.

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### Q: What is the unit circle and how does it relate to trigonometry?

The unit circle is a circle with a radius of 1 centered at the origin. It helps us visualize coordinates and angles in trigonometry.

### Q: How do we determine the coordinates of points on the unit circle?

The x-coordinate of a point on the unit circle is equal to the cosine of the angle, while the y-coordinate is equal to the sine of the angle.

### Q: How does the unit circle help us extend the definitions of trigonometric functions?

By using the coordinates of points on the unit circle, we can define cosine as the x-coordinate and sine as the y-coordinate. This allows us to evaluate trigonometric functions for any angle, not just right triangles.

### Q: What is the relationship between the unit circle and positive angles?

Positive angles are measured counterclockwise from the positive x-axis. The terminal side of an angle intersects the unit circle at a point whose coordinates represent the values of cosine and sine.

## Summary & Key Takeaways

• The unit circle is a circle with a radius of 1 and is centered at the origin. It helps us visualize coordinates and angles.

• Positive angles are measured counterclockwise from the positive x-axis, while negative angles are measured clockwise.

• The sine of an angle is equal to the y-coordinate of the point where the angle intersects the unit circle, and the cosine is equal to the x-coordinate.

• By using the unit circle, we can extend the definitions of trigonometric functions beyond right triangles.