Parts of a Circle

Let's start with a brief introduction:
Here's our circle:

We marked $M$ as the point at the center of the circle!

The radius is the distance from the center point of the circle to any point on its circumference.
Additionally the circle's radius equals half the circle's diameter, which is how we'll learn what a diameter is.
Radius is usually denoted by the letter $R$
and can meet any point on the circumference. As long as it extends from the circle's center to the circumference, it is called the circle's radius

Examples:

The diameter is a straight line that passes through the center point of the circle and connects $2$ points on the circumference. Note that the diameter must pass through the center point!
The diameter will always be equal to twice the radius because the radius is half of the diameter.
Let's look at some examples:

This is not an apple pie but rather pi in mathematics.
Pi is simply a constant number that represents the ratio between a circle's circumference and its diameter, and it appears in many formulas related to circles.
Its symbol is $π$ and it always equals $3.14$
All you need to remember is that when you see the pi symbol, you substitute $3.14$.

The perimeter is actually the total length of the circular line that surrounds the circle. To calculate the perimeter's value, we'll use a formula.
Circle perimeter formula:
$radius\cdot pi\cdot2=$

If we only have the diameter we can write that the circumference of the circle equals

$diameter\cdot pi$

Remember - Pi is a constant number and will always be $3.14$

A perpendicular line is a straight line that extends from the center of the circle to any chord in the circle.
* The perpendicular line divides the chord into two equal halves
* The perpendicular line creates $2$ right angles with the chord
* The perpendicular line bisects the arc (from the circumference) corresponding to the chord
Let's observe it in the illustration:

In other words, if we look at the illustration:
$AB=BC$
$AD=DC$
angle $MBC=90$
angle $ABD=90$

The area of a circle is calculated using the following formula:
$Pi\cdot r^2$
In words - to calculate the area of a circle, multiply pi times the radius of the circle squared
Remember - pi = $3.14$

And now let's practice! Ready?

Question –
If given a circle with a diameter of $18$ cm
What will be the radius of the circle?

Solution:
We learned that diameter is twice the radius. This means that if we divide the diameter by $2$ we will get the radius.
$18:2=9$
The radius equals $9$ cm

Additional question:
Given a circle with a diameter of $20$ cm.
Determine the circumference of the circle? Use the radius

Solution:
Let's remember how to calculate the circumference of a circle. The formula for the circumference of a circle is
$radius\cdot\pi\cdot2=$
The given diameter is $20$ cm, which means the radius is half of $20$, meaning:
$20:2=10$
Pi is always $3.14$
Now let's insert the data into the formula and we obtain the following answer:
$10\cdot3.14\cdot2=62.8$
The circumference of the circle is $62.8$ cm
Note - If we weren't asked to use the circle's radius, we could have used the diameter in order to achieve the same result, just remember not to include the $2$.
So according to the diameter formula, the circumference of the circle is:
$20\cdot3.14=62.8$
The result hasn't changed!