Sine, Cosine, Tangent and the Unit Circle
Let θ be an angle measured in radians drawn in standard position together with a unit circle. The radian measure of θ is the length of the arc on the unit circle subtended by the angle. The sine of the angle is the coordinate of the point where the terminal side of the angle intersects the unit circle, the cosine of the angle is the coordinate of this same point, and the tangent of the angle is the slope of the line passing through the same point and the origin. The graphs of sine, cosine, and tangent are created directly from this unit circle interpretation of the three functions.