Area of a Circle: Applying the formula

Look at the circle in the figure:

The radius is equal to 7.

What is the area of the circle?

Remember that the formula for the area of a circle is

πR²

We replace the data we know:

π7²

π49

49π

Given that the diameter of the circle is 7 cm

What is the area?

First we need the formula for the area of a circle:

 $\pi r^2$

In the question, we are given the diameter of the circle, but we still need the radius.

It is known that the radius is actually half of the diameter, therefore:

$r=7:2=3.5$

We substitute the value into the formula.

$\pi3.5^2=12.25\pi$

$12.25\pi$ cm².

O is the center of the circle in the diagram below.

What is its area?

Remember that the formula for the area of a circle is

πR²

We insert the known data:

π3²

π9

$9\pi$ cm²

Look at the circle in the figure:

The diameter of the circle is 13.

What is its area?

First, let's remember what the formula for the area of a circle is:

$S=\pi r^2$

The problem gives us the diameter, and we know that the radius is half of the diameter therefore:

$\frac{13}{2}=6.5$

We replace in the formula and solve:

$S=\pi\times6.5^2$

$S=42.25\pi$

42.25π

Given the semicircle:

What is the area?

Formula for the area of a circle:

$S=\pi r^2$

We complete the shape into a full circle and notice that 14 is the diameter.

A diameter is equal to 2 radii, so:$r=7$

We replace in the formula:$S=\pi\times7^2$

$S=49\pi$

24.5π

The center of the circle in the diagram is O.

What is the area of the circle?

$25\pi$ cm².

A circle has a diameter of 4 cm.

What is its area?

$4\pi$ cm²

Look at the circle in the figure:

$$

The radius of the circle is 4.

What is its area?

$16π$