Quote:
Originally Posted by ukorov  If you just want to prove that exterior angle COB = 2 x angle A,
consider a line AO and extend it to D.
AOD divides angle A into A1 (CAO) and A2 (BAO).
by theorem of exterior angles of triangles, you see that
angles COD = 2x A1 and angles BOD = 2 x A2
hence exterior angle COB = 2 x (A1 + A2) = 2A |
Thanks.
What I wanted to know was that, by the circle theorem, the interior angle COB should be 2A, the one forming the triangle. Not the one outside that they have taken.
Why is this? The one outside is not forming a triangle.