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Recently my differential geometry lecturer demonstrated that the sum of the interior angles of a triangle in a sphere is not necessarily never $180^\circ$. This is one way to prove that the earth is not flat. I was wondering, what then is the maximum sum of the interior angles of triangles in a sphere, since this sum is not a constant?

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    The maximal sum of interior angles is achieved by drawing a very small triangle somewhere on the sphere and then declaring the inside to be the outside and vice versa. The sum of the interior _and_ exterior angles is necessarily always $3\times 360^\circ$ and since one of these sets cannot sum to less than $180^\circ$, the opposite one cannot be _more_ than $5\times 180^\circ$.2011-10-24
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    The "not necessarily" in the question should be replaced by "never" (unless one is prepared to accept degenerate triangles, where all three vertices lie on a line).2011-10-25
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    @GerryMyerson Hmm...I never thought of that.2011-10-25

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Perhaps your teacher taught you something like this from Wikipedia: $$180^{\circ}\times\left(1+4 \tfrac{\text{Area of triangle}}{\text{Surface area of the sphere}}\right)$$

If you are prepared to have a triangle which has more than half the area of the sphere then the maximum can approach $900^\circ$; if not then $540^\circ$.

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    "Maximum can approach"? Does that mean the max doesn't exist, and I should be asking for the supremum instead? That seems to tie in with that Henning said earlier.2011-10-25
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    You get an angle sum close to $180^{\circ}$ with a very small triangle. Turn this inside out and you get an angle sum close to $900^{\circ}$ with a triangle which covers almost all of the sphere. In each case the limit is a degenerate triangle (as Gerry Myerson said) and so perhaps cannot be achieved.2011-10-25