Pythagoras
In a right triangle, the squares of the two short sides (the legs) add up to the square of the long side (the hypotenuse). If you know two of the sides, you can always find the third.
Your numbers
Answer
Side c
5cm
How to work it out
- 1a² + b² = c²
- 23² + 4² = c²
- 39 + 16 = c²
- 4c² = 25
- 5c = √25 = 5 cm
The formula
a² + b² = c²
Only applies in a right triangle. c is the hypotenuse — the long side opposite the right angle.
c = √(a² + b²)
When you know both legs.
a = √(c² − b²)
When you know the hypotenuse and one leg.
Tip from the workshop: the 3-4-5 rule. Measure 3 one way and 4 the other — if the diagonal is exactly 5, the corner is square.
More: Geometry
- Area and perimeterRectangle, triangle, parallelogram, trapezoid and circle — area and perimeter.
- Volume and surface areaBox, cube, cylinder, cone, sphere and pyramid — also in litres.
- The circleRadius, diameter, circumference and area from one known measurement — and sectors.
- Right triangleSine, cosine and tangent: all sides and angles from two known values.
- AnglesThe third angle in a triangle and the angle sum of a polygon.
- Points in the coordinate systemDistance, midpoint and the line between two points — drawn in.