circle question

Arc ACB is half the circle.  The whole circle is 360^o so ACB is 180^o   

Arc AC is given as 100^o which leaves 80^o for arc CB   

There's a relationship between the size of an angle subtended by an arc of a circle.  

When the angle's vertex lies on the circle the size of the angle is equal to half the arc.  

So angle CAB is 40^o   

BTW, when the angle's vertex lies on the center of the circle, the arc and the angle are the same amount of degrees. 

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