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This level of abstraction occurred in ancient Greece in the 6th century B.C.
Thales contributed geometric proofs. The Pythagoras school gave us the Pythagorean thereom, which relates to trigonometry. This theorem states that for every right-angled triangle, with sides A and B and hypotenuse C, there is a relationship between the two sides and the hypotenuse, such that: A2+B2=C2.
Proof:
Let the red triangle be the right-angled triangle. The yellow triangles each have area AB/2, and the blue square has area (B-A)2. Thus C2=2AB+(B-A)2=A2+B2.
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