In the diagram, a is the center of the circle, A,b and C are points on the circle.

Angle COA = 130

i) Find the size of angle CBA

I know it's 65 because it's half of CaA (O doesn't work in paint) but what is this law called? What is the CaA angle called and the CBA angle called?

This is all GCSE standard, please do not venture further.

P.S that angle reads '230'