A right-angled triangle has one square corner — a right angle. The two sides meeting at that corner are the legs; the longest side, opposite the right angle, is the hypotenuse.

Pythagoras' theorem says that if you build a square on each side, the two smaller squares together cover exactly the same area as the big one:

For example, a triangle with legs 3 and 4 has a hypotenuse of 5, because 3^2 + 4^2 = 9 + 16 = 25 = 5^2.

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