Every right-angled triangle has the same three named parts. Learn to point at them instantly and the theorem more or less writes itself.
The Three Parts
The right angle — the square corner, usually shown with a little box. The hypotenuse — the longest side, and it always sits directly opposite the right angle. The two legs — the other two sides, the ones that meet to make the right angle. We label the legs a and b, and the hypotenuse c.
State the Theorem
With the parts named, the rule is just one line: a² + b² = c². In words: the square of the hypotenuse equals the sum of the squares of the other two sides. The hypotenuse has to be the c — the lone term on its own side of the equals sign — because it's the biggest square.
A Worked One — Just Reading It
Picture a right triangle with legs of 6 and 8 and a slanted side of 10. Which is the hypotenuse? The 10 — it's the longest and it faces the square corner. So a = 6, b = 8, c = 10, and the theorem reads 6² + 8² = 10², i.e. 36 + 64 = 100. Notice we haven't solved for anything — we've just identified the parts and written the rule correctly. That's the whole job on this rung.
Why Getting C Right Matters
Put the wrong side as c and the equation is simply false. The hypotenuse earns its lonely spot because its square is the big one the other two add up to. Naming first, formula second — never the other way round.