Step by step
The question
Question: D E is parallel to B C, with AD = 6 cm, AE = 4 cm, EC = 6 cm: find DB.
1 Use the parallel line
D E is parallel to B C: the arrows show it.
So triangle A D E has the same angles as A B C: they are similar.
A parallel line cuts the two sides in the same ratio: A D to D B is the same as A E to E C.
2 Write the proportion
So: AD over DB equals AE over EC.
Careful!
Keep the same order on both sides: the matching parts on top, the matching parts at the bottom.
Put the numbers in.
We call DB x: 6/x = 4/6.
3 Cross multiply
When two fractions are equal, we can cross multiply.
These two are equal.
Work out the multiplications.
Let's swap the two sides, so x is on the left.
4 Remove the 4
4x means 4 times x.
We want just x.
To remove times 4, we divide by 4.
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.
4 divided by 4 is 1.
So just x is left.
36 divided by 4 is 9.
So x = 9!
5 Check
Let's check: put 9 in place of x.
First the left side.
Now the right side.
Both sides are 2/3.
So x = 9 is right!
So DB = 9 cm.
Let's look back.
First: use the parallel line.
Then: write the proportion.
Then: cross multiply.
Then: remove the 4.
Last: check.