Step by step
The question
Question: find the area of a trapezoid with parallel sides 5 and 8 and height 3.
1 Two make a parallelogram
A trapezoid has one pair of parallel sides: the top and the bottom.
In some countries it is called a trapezium.
Take the same trapezoid again.
Turn it upside down and put it next to the first one.
Together they make a parallelogram.
Its base is the bottom of one and the top of the other: 8 + 5.
Our trapezoid is half of that parallelogram.
2 Use the rule
A parallelogram is base times height.
We take half: A = ½ × (a + b) × h.
Now put the numbers in: a = 5, b = 8 and h = 3.
Brackets first: 5 plus 8 is 13.
13 times 3 is 39.
Now half of it.
Half of 39 means 39 divided by 2.
That is 19.5.
So the area is 19.5 square units.
3 Check
Let's check another way.
A line from corner to corner cuts it into two triangles.
The bottom triangle has base 8.
The top one has base 5.
Both have height 3.
Each is half of base times height.
We get 19.5 again.
So the area is right!
Let's look back.
First: two make a parallelogram.
Then: use the rule.
Last: check.