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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?
• Yeah. If you do the same thing, but change the measurement, you can do the same thing for inches like you did with meters.
• What if the units don't fit in the real shape?
• 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).
• What is a meter?
• A meter is a unit of measurement in the metric system it is about 39 inches, so a little longer than a yard.
• Does it matter if it's a rectangle or a square in area? Wouldn't it still be length times width?
• 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.
• can i use this information for finding area in triangles?
• No, you use area by multiplying base times height, then divide by 2 by using square units.
• 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?
• 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.
• How is a square a rectangle?
• 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.
• I'm a Chinese-speaker student. At , 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.
• 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!
• 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?