Any right-angled triangle
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Right triangles - those with a 90-degree angle.
Obtuse
Although the Pythagorean theorem (sums of square of a right angled triangle) is called a theorem it has many mathematical proofs (including the recent proof of Fermats last theorem which tangentially also prooves Pythagorean theorem). In fact Pythagorean theorem is an 'axiom', a kind of 'super law'. It doesn't matter if anyone does oppose it, it is one of the few fundamental truths of the universe.
Your question is so confusing that I almost trashed it and am not sure yet what you want to know but I have a possible idea : consider a right triangle each of whose legs have length 1. By the Pythagorean theorem, the hypotenuse has length equal to the square root of 2. The square root of 2 is irrational- one can prove it is not equal to any fraction of integers, yet it is obviously is a number of some kind. Thus the number system had to be extended to include numbers of this kind.
They can be any kind of triangles.