# simple geometry proof about circle

Let points $A, B$ and $C$ be on a circle $O$ such that segment $AB$ is a diameter of $O$ (meaning the center of the circle is the midpoint of the segment). Prove $m∠C = \frac{1}{2}$ (Internal angle sum(Δ$ABC$)).

Now this is not in Euclidean geometry and it is in neutral geometry. I don't know how to prove it and plus I'm limited since I haven't proven that the sum of the angles in a triangle add up to $180$ and the exterior angle inequality where the angle outside a triangle adds up to the two interior angles. Instead all I have is the Sacceri Legendre theorem(all angles of a triangle add up less than $180^{\circ}$ and exterior angles( angle on the outside is greater than either interior angles). I can use everything else.

Here is a picture:

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"Neutral geometry" is simply Euclidean geometry but without the parallel postulate. Is this correct? – Fly by Night Feb 12 '13 at 21:44
No but its the same as absolute geometry. But im also limited since i can't use the the theorem that says angles in a triangle add up to 180 or the exterior angle theorem. – user60887 Feb 12 '13 at 21:46
According to Wikipedia "Neutral geometry" is another name for "absolute geometry". – Fly by Night Feb 12 '13 at 21:49
yes. Now how would i go about it? I tried drawing a tangent line through point c and a line from point c to the center R. But thats as far as i got. – user60887 Feb 12 '13 at 21:52
One minute "no", next minute "yes". – Fly by Night Feb 12 '13 at 21:53

Draw the line from the center of the circle to $C$. The two triangles are isoceles, so $m \angle C = m \angle A + m \angle B$
@user60887: $A, B,C$ are on the circle, so look at radii. – Ross Millikan Feb 12 '13 at 22:02
Now $\angle RAC =\angle RCA$ – Ross Millikan Feb 12 '13 at 22:17