TRIANGLE: Now he's 4 cm high and 3 cm wide, like me. He's 12 square cm, so I must be half that… 6 square centimetres! And whatever shape I am it still works! RECTANGLE: You're just really embarrassing ...

Save guides, add subjects and pick up where you left off with your BBC account. The base and the perpendicular height are at right angles to each other. The measurements used must be in the same units ...