Before you see this post,
see
[Pic. 1]
Use The parallel postulate, we can find those facts in [Pic. 1]
[Pic. 2]
There is a triangle.
0°<{α,β,γ}<180°
[Pic. 3]
We can draw like [Pic. 3] by
[Pic. 4]
let's draw l4 is
parallel with l2.
[Pic. 5]
We can find sum of angles of the triangle by
facts in [Pic. 1].
α+β+γ = 180°
So, sum of angles of the triangle is 180°.
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