# Determining the measure of a central angle in a circle ( subject math GRE exam 9768 Q.23).

The question is given in the following picture:

I know that the triangle ABC will be an isosceles triangle but I do not know how to use this knowledge in the solution, could anyone help me please ?

• What shape are the triangles $OAB$ and $OAC$? Jul 22 '17 at 14:41
• @LordSharktheUnknown equilateral triangles. yeah I got it. Jul 22 '17 at 14:45
• So the quadrilateral $OBAC$ is two equilateral triangles stuck together. Jul 22 '17 at 14:46
• Could you please name your questions so they refer to the topic they are about instead of impenetrable reference codes for where in some unspecified filing system you found the problem that prompted you to ask? Thank you. Jul 22 '17 at 14:49
• ok I will do @HenningMakholm Jul 22 '17 at 14:50

Let the radius of the two circles to be $r$, then $OA=OB=OC=r=AO=AB=AC$