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In this lesson, you will learn what a reference angle is and why they are important in trigonometry, precalculus, and calculus. A reference angle is the angle measure from the terminal ray of the angle to the x-axis. It is always a positive angle. The reason we use reference angles is because the sine and cosine (sin & cos) of any angle around the unit circle can be found by examining the corresponding reference angle. In other words, the sine and cosine ratios are the same values all around the unit circle, and the magnitude of this value only depends on the reference angle. We then apply the correct sign to the final answer depending on what quadrant the original angle is located. In this lesson, we will draw an angle, find the reference angle, and discuss how to find the sine and cosine of the angle using its corresponding reference angle.