Step by step
The question
Question: find the surface area of a sphere (a ball) with diameter 18 cm.
1 Find the radius
The diameter goes all the way across the circle.
The radius is half of it.
Half of 18 is 18 divided by 2.
18 divided by 2 is 9.
So r = 9 cm.
2 Use the rule
Cut the ball in half.
The flat circle inside has area π × r².
The skin of the ball covers exactly 4 of these circles.
Like the peel of an orange.
π is a special number for circles.
It is about 3.14.
So the surface area is S = 4 × π × r².
Now put the numbers in: r = 9.
9² means 9 times 9.
That is 81.
So S = 4 × π × 81.
4 times 81 is 324.
So S = 324π.
We keep π in the answer.
That is the exact answer.
3 About how much
To see about how much, put 3.14 for π.
324 times 3.14 is 1017.36.
So the surface area is about 1017.36 cm².
Let's look back.
First: find the radius.
Then: use the rule.
Last: about how much.