The following code will generate the figure above.

Input := 


c = ParametricPlot[{Cos[t],Sin[t]},{t,Pi/6,2 Pi - Pi/4},

			Axes->False, AspectRatio->Automatic]

Input := 


l1 = Graphics[Line[{{0,0}, {Cos[Pi/6], Sin[Pi/6]}}]]

Input := 


l2 = Graphics[Line[{{0,0}, 

		{Cos[2 Pi - Pi/4], Sin[2 Pi - Pi/4]}}]]

Input := 


let = Show[Graphics[{Text["Angle theta",{.5,0}],

					Text["Radius r",{.4,.45}]}]]

Input := 


Show[l1,l2,c,let,AspectRatio->Automatic]