① Draw an arc with O as the center and any length as the radius, and intersect OA and OB at points C and D respectively;

(2) Make a ray O'B', draw an arc with o' as the center and OC length as the radius, and intersect with O'B' at point c';

③ Draw an arc with C' as the center and CD length as the radius, and the arc before intersection is at D';

(4) make ray o'. A' after point d'.

So ∠A'O'B' is the angle equal to ∠AOB.

In △ o ′ c ′ d ′ and △OCD,

o′C′= OC

Outside diameter = outside diameter

c′D′= CD

∴△o′c′d′≌△ocd(sss)，

∴∠a′o′b′=∠aob，