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### Course: Measurement & Data - Statistics & Probability 189-200 > Unit 1

Lesson 4: Multiply to find area# Counting unit squares to find area formula

Sal uses unit squares to see why multiplying side-lengths can also find the area of rectangles. Created by Sal Khan.

## Want to join the conversation?

- Could I use this information for inches as well?(32 votes)
- Yeah. If you do the same thing, but change the measurement, you can do the same thing for inches like you did with meters.(35 votes)

- What if the units don't fit in the real shape?(0 votes)
- If the units don't fit exactly, then you need to use fractions of the unit. For example, if I draw a straight line and it is more than three inches long, but shorter than four inches, then it would be 3 and
**something**inches long (for example 3 1/2 inches).(4 votes)

- What is a meter?(0 votes)
- A meter is a unit of measurement in the metric system it is about 39 inches, so a little longer than a yard.(2 votes)

- Does it matter if it's a rectangle or a square in area? Wouldn't it still be length times width?(54 votes)
- That's true. The only thing is, when calculating the area of a square, you can simply multiply one of the sides by itself, since you know the other sides are all the same.(28 votes)

- can i use this information for finding area in triangles?(7 votes)
- No, you use area by multiplying base times height, then divide by 2 by using square units.(8 votes)

- If a rectangle has width of 0.5cm and length of 0.6cm does the area equal to 0.3cm^2? and if it's true, how is this area less than width and length?(4 votes)
- Yes. That's right and thought-provoking. You can't really compare the length or width to the area. One is cm and the other is cm^2. It's impossible to say if a pencil is longer than an hour. It's asking you to compare time to length, which isn't possible. The same thing happens in this case.(5 votes)

- How is a square a rectangle?(2 votes)
- A rectangle is a quadrilateral (A shape with 4 sides) in which all 4 angles are right angles; opposite sides parallel and equal.

A square meets all of these criteria, thus a square is also a rectangle.(5 votes)

- I'm a Chinese-speaker student. At04:44, I can't understand the sentence that "Just fill it really good about multiplying the dimensions of this rectangles." Thank you who might help me overcome this language hatch very much! Although it's hard for me to watch videos at Khanacademy. org in English, I've gained a lot of fun and knowledge that I've never acquired in my classroom in China.(3 votes)
- What he said was "just to feel really good about multiplying the dimensions of these rectangles." This means that he wants to make sure we are comfortable multiplying the dimensions of rectangles, or he wants to make sure we know how to do it, why to do it, and why it works.

I hope this helps!(3 votes)

- When we're doing this, how do you find the area if one of the perimeters is a variable, and you have to figure out the variable first?(2 votes)
- Well, first there are two terms that sound similar but have different meanings. The perimeter of a shape ( or polygon) is the distance around the edge. A parameter is part of a function (or formula), similar to a variable, but it is held constant for a given problem.

So if you are asked to find the area of a rectangle and you are given the length as a measurement, say 5m, and you are given the width as a variable, say W, then you can still multiply these two values to get: Area = 5 x W = 5W square meters. Once you find out what W is, you can plug it in to the equation to find the area.(2 votes)

- what if it is a trapiziod or other shapes?(2 votes)
- If it is a trapezoid or other shape other than a triangle, square, or rectangle, then you will need different sorts of formulas, such as when working with circles you need πr^2 instead of a "base times height" when what you have is a square, rectangle, or other parallelograms. Once you get into higher levels of math, you might learn about Heron's Formula, Integration, the Trapezoidal Rule, and other things (that are taught in Trigonometry and Calculus).(2 votes)

## Video transcript

I've got three rectangles
here, and I also have their dimensions. I have their height
and their width. And in fact, this one right here
has the same height and width, so this is actually a square. So let's think about how
much space they each take up on my screen. And since we're doing
everything in terms of meters, since all of the
dimensions are in meters, I'm going to measure the area
in terms of square meters. So let's see how
many square meters I can fit onto this
yellow rectangle without going outside
of its boundary and without overlapping. So I can fit 1 square meter. Remember, a square
meter is just a square where its length is 1 meter
and its width is 1 meter. So that's 1 square meter, 2, 3,
4, or 5, and 6 square meters. So we see here that the
area is 6 square meters. Area is equal to
6 square meters. But something might
be jumping out at you. Did I really have to sit
and count 1, 2, 3, 4, 5, 6? You might have recognized
that I could view this as really 2 groups of 3. And let me make that very clear. So, for example, I could
view this as one group of 3 and then another group of 3. Now, how did I get groups of 3? Well, that's because
width here is 3 meters. So I could put 3 square
meters side by side. And how did I get the 2 groups? Well, this has a
length of 2 meters. So another way that I
could have essentially counted these six things
is I could have said, look, I have a
length of 2 meters. So I'm going to
have 2 groups of 3. So I could multiply 2 times
3, 2 of my groups of 3, and I would have gotten 6. And you might say, hey, wait. Is this just a coincidence
that if I took the length and I multiplied
it by the width, that I get the same
thing as its area? And no, it's not, because
when you took the length, you essentially said, well,
how many rows do I have? And then you say when you
multiply it by the width, you're saying, well,
how many of these square meters can I fit into a row? So this is really a
quick way of counting how many of these
square meters you have. So you could say that 2
meters multiplied by 3 meters is equal to 6 square meters. Now, you might say, hey, I'm
not sure if that always applies. Let's see if it applies
to these other rectangles right over here. So based on what
we just saw, let's take the length, 4 meters,
and multiply by the width, and multiply by 2 meters. Now, 4 times 2 is 8. So this should give
us 8 square meters. Let's see if this is
actually the case. So 1, 2, 3, 4, 5--
and you see it's going in the right
direction-- 6, 7, and 8. So the area of this rectangle
is, indeed, 8 square meters. And you could view
this as 4 groups of 2. So you could literally
view this as 4 groups of 2. That's where the 4
times 2 comes from. So you could view it as
4 groups of 2 like this. Or you could view
it as 2 groups of 4, So 1 group of 4 right over here. So you could view this is 2
times 4, and then 2 groups 4. I want to draw it a
little bit cleaner. Now, you could
probably figure out what the area of
this rectangle is. It's actually a
square, because it has the same length
and the same width. We multiply the length, 3
meters, times the width, so times 3 meters, to get 3
times 3 is 9-- 9 square meters. And let's verify it again just
to feel really good about this multiplying the dimensions
of these rectangles. So we have 1, 2, 3,
4, 5, 6, 7, 8, and 9. So it matches up. We figure out how many
square meters can we cover this thing with,
without overlapping, without going over
the boundaries. We get the exact same thing
as if we multiplied 3 times 3, if we multiplied the length
times the width in meters.