A right triangle has one angle of exactly 90° — marked with a little square. The side facing that right angle is the hypotenuse, and it's always the longest side. The other two are the legs.
Drag the corners
isosceles · acute
angles sum ≈ 180°
Why right triangles are a big deal
- Pythagoras lives here: leg² + leg² = hypotenuse², i.e. a² + b² = c².
- All of trigonometry is built on them — sin, cos and tan are just ratios of a right triangle's sides (SOH-CAH-TOA).
- The two non-right angles always add to 90° (they're complementary), since the three angles total 180°.
- Drop a perpendicular from the right angle to the hypotenuse and you split it into two smaller right triangles, both similar to the original.
Your turn
A right triangle has legs 6 and 8. How long is the hypotenuse?
Watch out
Pythagoras only works for right triangles, and c must be the hypotenuse (the longest side, opposite the 90°). Plugging the wrong side in as c gives nonsense.