2) 0 degrees and 180 degrees.

How to find it?

Dave, I want you to draw a circle with a radius of 1 on the graph, and observe. Plot the primary points: For 0 degrees: (1, 0), for 90 degrees: (0, 1), for 180 degrees: (-1, 0), for 270 degrees: (0,-1). Then join these 4 points to form a circle. Remember that the points are actually P(cos(x), sin(x)). What do you see at 0 degrees? The points are (1, 0). So, cos (0)=1, and sin(0) =0.

cot(theta) = cos(theta)/sin(theta) = 1/0 = Infinity