In the diagram below, AB is the chord of a circle with centre O. OM is perpendicular to AB (meet at a right angle). Look at triangles OAM and OBM. The hypotenuses (OA and OB) are the same ...

To find the area of a right angled triangle, you need to multiply the two perpendicular sides together and halve the result. On occasion, you may need to find the area of more irregular shapes ...