Step by step
The question
Question: the radius is 7 cm and the chord is 4 cm from the center: find the length of the chord AB.
1 Use the chord rule
The line from the center at 90° to a chord cuts the chord in half.
M is the middle of A B.
So O, M and A make a right triangle.
The right angle is at M.
O A goes from the center to the circle: it is a radius.
2 Name the sides
The longest side is called the hypotenuse.
We call it c.
It is always across from the right angle.
The two shorter sides make the right angle.
We call them a and b.
The rule for every right triangle: a² + b² = c².
3 Put the numbers in
Now put the numbers in: a = 4 and c = 7.
4 Work out the squares
4² means 4 times 4.
That is 16.
7² means 7 times 7.
That is 49.
Write the squares: 16 + b² = 49.
5 Remove the 16
We want b² by itself.
The 16 is in the way.
To remove 16, we take away 16.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
16 and − 16 make zero.
Cross them out.
49 minus 16 is 33.
6 Take the square root
b² is 33.
So b times b is 33.
That means b is the square root of 33.
33 is not a square number.
So we keep √33.
That is the exact answer.
It is between 5 and 6.
With a calculator, √33 is about 5.7, to the nearest tenth.
So b is about 5.7 cm.
7 Double it
The chord is twice A M: about 11.5 cm.
8 Check
Let's check: put b = √33 back.
First a² + b².
Now c².
√33 times √33 is 33.
Both are 49.
So b = √33 is right!
Let's look back.
First: use the chord rule.
Then: name the sides.
Then: put the numbers in.
Then: work out the squares.
Then: remove the 16.
Then: take the square root.
Then: double it.
Last: check.