In the diagram below, 
and 
lie on the circle with centre 
. If 
and 
, determine with reasoning 
and 
We know 
– radii of the circle.
Which means, 
is isosceles and 
– equal angles isosceles triangle.
– angle at the centre twice the angle at the circumference.
This means 
– angles on a straight line are supplementary
– equal angles isosceles triangle and the angle sum of a triangle.
– angle at the circumference subtended by the same arc are congruent.
– angles at the circumference subtended by the same arc are congruent.
– equal angle isosceles triangle
Hence 