Why do this problem?
Geometry sometimes offers surprising results that can be proved very elegantly. This is a great example of how finding the right representation (in this case a diagram) allows a problem to be solved more easily.
Show students the image:
Invite them to draw it and measure angles a, b and c to verify that a+b=c, and give them some time to think about how they might prove it. There are many methods that use advanced techniques such as trigonometry or vectors, but there is also an elegant geometric method. Once they have had time to engage with the difficulty of the problem, show them the following image:
"Can you find angles in the diagram that are the same size as angles a and b?"
"What can you say about the angle at B?"
"What can you say about the lengths AB and BC?"
"What can you deduce about the angles at A and C?"
Students may be interested to read 8 Methods for Three by One
which outlines a number of different methods for this proof using more advanced mathematics.
Angles in Polygons Short Problems
worksheets offer some more straightforward examples of geometrical reasoning problems.