I'm given the question:

"Explain why cos20 > cos50. Use a diagram in your explanation."

The 20 and 50 have a degree symbol.

I don't understand how to explain it. Can anyone help?

Graph the cosine function. Most calculators will use radians. However look at the graph between 0(0 degrees) and pi(180 degrees). You will see that the graph is decreasing. So cos(10)>cos(45)>cos(75)>cos(155) etc. BUT at 180 the graph will begin to increase.