# Circle: radius, diameter, circumference and area

A circle is a shape consisting of all points are the same distance from the center.

The **radius** is the distance between any point of the circle and the centre. The **diameter** is a line segment which goes straight across the circle and passes through the centre. The diameter is twice the radius: $D=2R$. The **circumference** is the distance around the circle. It can be calculated by these formulas below:

The **area** enclosed by a circle can be calculated by this formula below:

## Solved examples

What is the circumference of a circle with radius of 3.5 cm? Round your answer to the nearest one.

$$C=2\pi R=2\cdot 3.14\cdot 3.5 \approx 22 \text{ cm}$$What is the area of a circle with diameter of 5 cm? Round your answer to the nearest one.

$$A=\pi R^2=3.14\cdot \left (\frac{5}{2} \right )^2 \approx 20 \text{ cm}^2$$What is the area of a circle with circumference of 30 cm? Round your answer to the nearest one.

$$C=2\pi R, \ \ 30=2\pi R, \ \ R=\frac{15}{\pi}$$ $$A=\pi R^2=3.14\cdot \left (\frac{15}{3.14} \right )^2 \approx 72 \text{ cm}^2$$## Wanna check your skills?

A quiz is based on this article and has 5 questions.

### Quiz

2021-08-24

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