Hi Carlin,
In the diagram above C is the centre of the circle. The octagon can be divided into 8 triangles congruent to ACB and hence the measure of the angle ACB is ^{360}/_{8} = 45 degrees. Thus the triangle ACB is isosceles, with two sides of length 15.8 cm each and the angle between them measuring 45 degrees. Can you find the length of the third side?
Write back if you are still stuck.
Penny
