semicircle

Everyday

1. Half a circle: a shape made by cutting a circle in half along its diameter. tree →
2. Things arranged in a curved line like half a circle: «sit in a semicircle». tree →

In the lessons

Maths: a semicircle has a curved edge (half the circumference) and a straight edge (the diameter). Its area is half the circle's area: ½ × π × r². Its perimeter is half the circumference plus the diameter: πr + 2r. In class: «Sit in a semicircle on the carpet.» A protractor is a semicircle marked from 0° to 180°. In Alice, the animals sit in a ring for the Caucus-race.

Examples

• Draw a semicircle with a compass. tree →
• The children sat in a semicircle around the teacher. tree →
• A protractor is shaped like a semicircle. tree →
• The area of a semicircle is half the area of the full circle. tree →
• A rainbow looks like a semicircle in the sky. tree →
• If the radius is 2 cm, the semicircle's curved edge is about 6.28 cm. tree →

Note

«Semi-» means half. Countable. Maths trap: the perimeter is not just half the circumference – you must add the diameter.