1, circumference of a circle: The length of the curve around a circle is called the circumference of a circle.

2.Pi:

The ratio of the circumference to the diameter of any circle is a fixed number, which we call pi. Represented by the letter π. Pi π is an infinite acyclic decimal. In the calculation, π ≈ 3. 14 is generally taken.

3. The circumference formula of a circle: c =πd→d = c \uπor.

C=2πr → r = C ÷ 2π

C half =πr+2r=5. 14r

practise

(1) The figure formed by the minute hand of the clock turning once is ().

(2) The line segment from () to () is called the radius.

(3) The line segment passing through () and both () is called the diameter.

(4) In the same circle, all radii () are equal to all (), and the diameter is equal to the radius ().

(5) Draw a circle with a compass with a diameter of 20 cm, and the distance between two steps of the compass is () cm.