You're lost. You walk 3 km north, then 4 km east. How far are you from your start? Pythagoras to the rescue.
Distance
√(3² + 4²) = √25 = 5 km
Triangle 3-4-5 is the most famous right triangle. The Egyptians used a knotted rope of 12 segments to make perfect 90° angles for the pyramids.