Main content

## 3rd grade

### Unit 10: Lesson 6

Decompose figures to find area# Decomposing shapes to find area: add

Lindsay finds the area of an irregular shape by decomposing it into 3 rectangles and adding the area of the rectangles. Created by Lindsay Spears.

## Want to join the conversation?

- do we learn decomposing homeschool?(11 votes)
- Yes, you can learn decomposing while being homeschooled.(3 votes)

- is m for meters ?(11 votes)
- Yes, m stands for meters, like how ft stands for feet.(3 votes)

- why does she use 6(3 votes)
- I presume you mean the 6m at the bottom.

6m is just the side length of the figure. However, how the area was calculated the 6m was divided into two part (3m each). So the 6m was used as 3m (times 9m) and 3m (times 3m).(8 votes)

- Can we not watch the video? I mean I know I have to learn but I'm in 6th grade not 3rd! No offense teacher.(4 votes)
- I'm a third grader be nicer(1 vote)

- can we do it with out decomposing?(3 votes)
- Yes, but it is _*very*_complicated.(1 vote)

- Its not the video i want?(3 votes)
- How do you know when to add 3.14 to the circle and when not to?(3 votes)
- Why didn't she add 15 & 27 vertically instead of horizontally? Why confuse kids more than they have to be?(2 votes)
- What happened to the 6meters at the bottom.(2 votes)
- Yes. If you're talking about length, a unit of m is short for a meter.(3 votes)

## Video transcript

- [Voiceover] What is
the area of the figure? So down here we have this one, two, three, four, five, six, seven,
eight, nine, 10-sided figure, and we want to know its area, how many square meters
does this figure cover? And we have some measurements, that seems helpful, but what's not too helpful to me is I don't know the special trick to find the area of a 10-sided figure so I've got to think about what I do know and what I do know is the way to find area of a rectangle. So what I can do, because I can see, if I can find any rectangles in here. Here's one rectangle, right there. So I can find the area of that part. Then let's see if I can find any more. Here's another rectangle. So I can find the area of that part. We could call that one
a rectangle or a square. And then that leaves
us with this last part, which is again, a rectangle. So what we did is, we broke this up or decomposed it into three rectangles and now if I find out how much space this purple one covers, and the blue one and the green one, if I combine those, that
would tell me the area of the entire figure, how much space the entire figure covers. So let's start with this one right here. This one is three meters long, so we can kind of divide
that by three meters, into three equal meters, and then we've got a width
of two meters down here so we can split that in half. So if we draw those lines out, we can see this top row is
going to cover one square meter, two square meters, three square meters, and then there's two rows of that, so there's two rows of three square meters for a total of six square meters. This rectangle covers six square meters, so this part of the entire figure covers six square meters. The next one, our measurements
are three and a three, so it will have three rows
of three square meters or nine square meters, and then finally this purple one has three meters and nine, so we can say it will
have three rows of nine or nine rows of three square meters which is 27 square meters. So the area of this purple section, it covers completely 27 square meters. The green covers nine square meters, and the blue covered six square meters. So, if we combine all those areas, all those square meters it covers, that will tell us the
area of the entire figure. So we have six square meters, plus nine square meters, plus 27, and we can solve that,
six plus nine is 15, 15 plus 27, let's see, five ones and seven ones is 12 ones. We'll just find some space up there. And one 10 and two 10's or a 10 and a 20 is 30. And 30 plus 12 is 42. So the area of the entire figure is 42 square meters.