Problem

We take a circle and inscribe a triangle in it.

Complete the steps of the table below to find an interesting relationship between the angles and sides of this triangle.

Diagram Description
Triangle ABC inscribed in a circle
1. Add a point \(C'\) on the circumference of the circle, so that \(\angle C'AB\) is a right-angle.

2. What is the relationship between angles \(ACB\) and \(AC'B\)

3. What is the length \(C'B\)?
Triangle ABC inscribed in a circle
4. Label side \(AB\) as \(c\). Write a relationship between \(c\) and the angle at \(C'\).
Triangle ABC inscribed in a circle
5. Can you therefore write a relationship between \(c\) and the angle at \(C\)?
Triangle ABC inscribed in a circle
6. Starting from the original diagram, add a point \(B'\) on the circumference of the circle, so that \(\angle B'CA\) is a right-angle.

7. What is the relationship between angles \(ABC\) and \(AB'C\)?

8. What is the length \(B'A\)?
Triangle ABC inscribed in a circle
9. Label side \(AC\) as \(b\). Write a relationship between \(b\) and the angle at \(B'\).
Triangle ABC inscribed in a circle
10. Therefore write a relationship between \(b\) and the angle at \(B\).
Triangle ABC inscribed in a circle with sides labelled a,b,c
11. In general, what can we now say about the relationship between the sides and angles of a triangle?

You have proved your statement for this particular triangle, which has its three vertices on the circumference of a circle. Is your statement true for all triangles?

If you are not sure, you might like to look at Finding circles.