On a circle of radius 1, an angle measured in radians is the length of the arc it cuts off, so a full turn is 2π. The point where the angle's arm meets the circle is (cos θ, sin θ), which gives sine and cosine for every angle, counterclockwise, negative or beyond one turn. The special triangles give exact values, and because the point sits on the circle, sin²θ + cos²θ = 1.