This video gives an introduction to how we use the unit circle to define sine and cosine for angles greater than 90 degrees. This is just a conceptual discussion, and I don't go over any problems just yet. The main ideas are: 1) For every angle θ, the point on the unit circle is (cos θ, sin θ). 2) We can draw triangles to find out exactly what that point is. 3) Different quadrants affect the sign (negative/positive) of that point, and thus affect whether cos θ or sinθ are negative or positive. If you get these ideas in mind, then the next videos, containing more specific examples, will be helpful.