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### Course: Geometry (all content) > Unit 7

Lesson 7: Area of trapezoids & composite figures# Area of composite shapes

We can sometimes calculate the area of a complex shape by dividing it into smaller, more manageable parts. In this example, we can determine the area of two triangles, a rectangle, and a trapezoid, and then add up the areas of the four shapes to get the total area. Created by Sal Khan.

## Want to join the conversation?

- Can't you just think of the bottom part as a trapezoid? You can add 6.5 and 3.5 and divide by 2 and multiply that by 9 to get the bottom part. That would save you the hassle of finding the triangle's and the rectangle's area.(118 votes)
- There can be many ways to do composite figures, and your way is just as valid since you divide it into known shapes especially since the area of trapezoids is the first in this string of videos. For fun, I might start with a large 10 by 9 rectangle and take away the three triangles that are cut off from the corner 90 - 3.5 - 13.5 - 12.25 = 60.75.(21 votes)

- What if the shape has a half circle in it?(23 votes)
- Then use the circle formula and divide it by 2.(73 votes)

- "Say when I grow up, what is this useful for?"(25 votes)
- geometry is useful for architecture, design,renovation, building, art, and sometimes just daily activities. This information will also be helpful if you end up needing or wanting to help someone else that has curriculum revolving around this subject. There many uses for geometry in life.(13 votes)

- I don't understand how to do this can someone explain?!

-\'_'/-(26 votes)- So what he's saying is that you need to break up the shape to make it easy to divide(16 votes)

- can some one explain i'm not giving up but i want to can any one help. Thanks.(14 votes)
- So to find the area of an oddly shaped figure that you don't have a formula for, you split it into lots of smaller figures that you already know how to find the area of. Then you add them all together to find the total area of the original larger figure. Does that make sense? Let me know if there's anything you still don't understand.(13 votes)

- I am sooooo confused about why he did 1 half times 7(13 votes)
- Actually, he meant 1/2*7. That is the same as dividing something by two.(3 votes)

- im brainig so hard my think hurts.(13 votes)
- Is the label that important? (square units)(3 votes)
- It helps you know which is which. For example, 25m is a line, but 25m2 is a shape.(2 votes)

- am i slow, or did this video make no sense at all(7 votes)
- It's pretty easy. Just transform the shape into squares and rectangles by drawing lines, calculate their area, and add it all up. Easy!(11 votes)
- It depends on who you are and how you learn... Some people may not learn as quickly as you because what language they speak or their environment or their teacher so it may be easy for you because all those thing are good for you(3 votes)

## Video transcript

- [Instructor] So we have the
strange-looking shape here, and then were given
some of its dimensions. We know that this side right
over here has a length of 3.5, this side over here is 6.5, then we know from here to here is two, and then from here to here is seven, and then they're giving
us this dimension right over here is 3.5. And so given that, let's see if we can find the
area of this entire figure. And I encourage you to
pause the video right now and try this on your own. So I assume you've given a go at it and there might be a few things that jump out at you immediately. The first thing is that they
have these two triangles up here and they give us all
of the dimensions for 'em, or at least they give us the
base and the height for it, which is enough to figure out the area. If I had a triangle or
if I had a rectangle that was 2 X 3.5 or two wide, two units wide and 3.5 units high. If had a rectangle like that, we know that it would
have an area of 2 X 3.5. Now a triangle is just going to be, or especially a triangle like this, a right triangle is just gonna be half of a rectangle like this. We just care about half of its area. So this area is going to be 1/2 X 2 X 3.5. And 1/2 X 2 = 1, 1 X 3.5 is 3.5 square units. So the area of that part is
going to be 3.5 square units. Now let's think about the area of this triangle right over here. Well, once again, we
have its height as 3.5, its base is seven. So its area is going to be 1/2 X 7 X 3.5. 1/2 X 7 is 3.5 X 3.5, so this part is 3.5, and then I'm gonna multiply
that times 3.5 again. So let's figure out what
that product is equal to. So 3.5 X 3.5. 5 X 5 is 25. 3 X 5 is 15 + 2 is 17. Let's cross that out, move
one place over to the left. 3 X 5 is 15. 3 X 3 is 9 + 1 is 10. So that gets us to 5 +
0 is five, 7 + 5 is 12. Carry the one, 1 + 1 is
two, and we have a one. And we have two digits to
the right of the decimal. One, two, so we're gonna have two digits to the right of the decimal in the answer. So the area here is 12.25 square units. Now this region might seem
a little bit more maybe, maybe a little bit more
difficult because it's kind of a, it's this weird trapezoid-looking thing. But one thing that might pop out at you is that you can divide it very easily into a rectangle and a triangle. And we can actually
figure out the dimensions that we need to figure out
the areas of each of these. We know what the width of
this rectangle is, right? Or the length of this rectangle, whatever you want to call it. It's going to be two
units plus seven units. So this is going to be nine. We know that this distance is 3.5. So if this distance
right over here is 3.5, then this distance down here has to add up with 3.5 to 6.5. So this must be three. So now we can actually
figure out the area. So the area of this
rectangle is just going to be its height times
its length, or 9 X 3.5. 9 X 3.5. And one way you could do it, we could even try to do this in our head. This is gonna be 9 X 3 + 9 X 0.5. So 9 X 3 is 27. 9 X 0.5, that's just half of
nine, so it's gonna be 4.5. 27 + 4 would get us to 31. So that's going to be equal to 31.5. Or you could multiply it
out like this if you like. But the area of this region is 31.5. And then the area of this
triangle right over here, it's gonna be 9 X 3 X 1/2,
we're looking at a triangle. 9 X 3 is 27, 27 X 1/2 is 13.5. And so to find the area
of the entire thing, we just have to sum up these areas. So we have 31.5, 31.5 + 13.5, 13.5, plus 12.25, 12.25, plus 3.5, plus 3.5 here. So we just have a five
here in the hundredths. That's the only one. 5 + 5 is 10 + 7 is 17. 1 + 1 is 2 + 3 is 5 + 2 is 7 + 3 is 10. 1 + 3 is 4 + 1 is 5 + 1 is six. So we get a total area for this figure of 60.75 square units.