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Expii
Trigonometry: Pythagorean Identity - Expii
The Pythagorean identity says that sin²(x) + cos²(x) = 1 no matter what x you plug in. (It's a consequence of the Pythagorean theorem.) There are other variants too, like tan²(x) + 1 = sec²(x) when you divide both sides by cos²(x).