The Saccheri-Legendre Theorem

[jessica098]

I am supposed to prove that the angle sum of any convex quadrilateral is less than or equal to 360 degrees.

I was told to use the Saccheri-Legendre Theorem as a hint, which states that the angle sum of any triangle is less than or equal to 180 degrees.

I know that I can split a quadrilateral into two triangles, so can somebody help me apply the Saccheri-Legendre theorem to the quadrilateral (two triangles)??

Thanks!

 

[Denis]

Did you google that silly theorem?