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In this lesson, you will learn how to use the unit circle in radian angle measure. First, we review the concept of the unit circle, which is a circle of radius of 1 on an xy plane. The projection of any radius of the unit circle onto the x-axis is the cosine (cos) of the angle. The projection of the radius onto the y-axis is called the sine (sin) of the angle. We can find the tangent (tan) by dividing the sine by the cosine of the angle. We use the unit circle in radian measure because it is a much more natural measurement system in trigonometry, pre-calculus, and calculus.