Question

What is the circumference of a circle that has radius `7"cm"` ?

Answer 1

Circumference = 7`pi`cm

Explanation:

Circumference = `pi` x d
Diameter = 7
C = `pi` x 7
C = 7`pi` cm

Answer 2

`7 pi` or `"21.98 cm"` (approx)

Explanation:

`d = "7 cm"`

Then

`r = d/2 = "3.5 cm"`

`c = 2*pi*r`

`c = 2 * pi * "3.5 cm" = 7 pi` `"cm"`

If you want an exact answer, you can multiply `7` by the value of `pi`, that is `~~3.14`.

So

`7 * 3.14 ~~ "21.98 cm"`

Answer 3

The circumference is about `21cm`

Explanation:

The circumference of a circle is about `3` times longer than the diameter.

The actual formula is `C = pi xxD`

The value for `pi` is about `3.142...`

So if you just want an approximate answer, you could say that the circumference of the circle in question is about:

`" "7 xx3 = 21cm`

Answer 4

Approximately `22"cm"`. Exactly `7pi"cm"`

Explanation:

The ratio between the circumference of a circle and its diameter is `pi`. We commonly write `c = 2pir`, using the radius instead and doubling it.

About 1500 years ago, a Chinese mathematician named Zu Chongzhi (祖沖之) calculated `pi` to approximately `7` decimal places and identified two useful rational approximations. One is `22/7`, which is common in use and the other is `355/113 ~~ 3.1415929`, which is less well known but very accurate for the number of digits you have to remember.

In the given example, we are told that the diameter is `7"cm"`. So using the common approximation `pi ~~ 22/7`, we find that the circumference is approximately:

`22/7 * 7"cm" = 22"cm"`