Step by step
The question
Question: find the volume of a sphere (a ball) with radius 9.
1 Use the rule
Put the ball in a cylinder just as wide and just as high.
The ball fills exactly two thirds of that cylinder.
Worked out, that gives the rule for a ball.
The rule has r³: r times r times r.
Careful: it is not 3 times r.
π is a special number for circles.
It is about 3.14.
So the volume is V = 4/3 × π × r³.
Now put the numbers in: r = 9.
9³ means 9 times 9 times 9.
That is 729.
So V = 4/3 × π × 729.
Keep the numbers small: divide 729 by 3 first.
729 divided by 3 is 243.
So V = 4 × π × 243.
4 times 243 is 972.
So V = 972π.
We keep π in the answer.
That is the exact answer.
2 About how much
To see about how much, put 3.14 for π.
972 times 3.14 is 3052.08.
So the volume is about 3052.08 cubic units.
Let's look back.
First: use the rule.
Last: about how much.