Main content

### Course: 3rd grade > Unit 11

Lesson 1: Perimeter- Perimeter: introduction
- Perimeter of a shape
- Find perimeter by counting unit squares
- Find perimeter by counting units
- Finding perimeter when a side length is missing
- Find perimeter when given side lengths
- Finding missing side length when given perimeter
- Perimeter review

© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Finding missing side length when given perimeter

The video illustrates how to find the lengths of unknown sides on a shape by using the given perimeter and the lengths of known sides. Two examples are given, and in each case, the missing side lengths are determined to be 5 units. Created by Lindsay Spears.

## Want to join the conversation?

- i just wanted to ask that in the previous task you asked a question what is the perimeter of a pentagon which is of 5 units so pentagon are of many kinds just it need to have 5 sides so what kind of pentagon is it(28 votes)
- Yes it is not always necessary that all the sides of the pentagon are equal, but when it is mentioned as "regular" pentagon (which I think they would've in your task), then all the sides are equal.

Hope this helps :)(27 votes)

- If for example, the given lengths are 10,6,5,3,and1, but then one whole section is unknown. What would be the given length?(18 votes)
- Without knowing the total perimeter length it is impossible to determine length of the missing section.(7 votes)

- I'm in third grade at the begining of the school year it was hard but now it's pretty easy you'll get it.(11 votes)
- this really helps i always had trouble with these.(9 votes)
- Yes, they are very good at explaining what the problem is and what you need to do to get the answer. Go Khan Academy! They are very thorough...(3 votes)

- When they say the perimeter, just take the perimeter (24 the teacher's first example) and decrease it with the centimeters you already have: 6+5+4+4 = 19: 24cm-19cm = 5cm(2 votes)
- what do we have school?(2 votes)
- The last question can't you just write an equation 22 + 2x = 32, because one side is 11 and the opposite side is equal so we get 22 and we have 2 sides x that we need to know, and just solve you would get x = 5 and both sides are congruent so both missing sides would be 5.(2 votes)
- So we need to add everything and minus it with the whole no.?(2 votes)
- ok all uv the stuff on the video is no true I did it myself(2 votes)

## Video transcript

- [Voiceover] The perimeter of the figure is 24 centimeters. What is the length of the missing side? So we're told this figure down here has a perimeter of 24 centimeters. The perimeter is 24 centimeters. So what that tells us is that the distance around the entire outside
is 24 centimeters. But what we want to know
is what is the length of the missing side? Well, the missing side
is going to be the side we're not told, so it
will be this one here. Because we have this question mark. It's missing, it is not labeled. So we've got to figure out what number should go on this missing side so that the entire
length is 24 centimeters. So let's start with what we do know. We know that this top length, this top side, has a
length of six centimeters, plus, going down the left, has a length of five more centimeters. Plus across the bottom
is four more centimeters. And the last known side is
another four centimeters. So when we add those
plus this mystery side, we should get a perimeter of 24. The entire distance, all of this, the six, plus the five, plus
the four, plus the four, plus this mystery number should get us to a perimeter of 24. So let's add over here and try
to make it a little simpler to find our missing side. I can add in any order,
and I love making tens, so I'm gonna say six and four is 10. 10 plus five is 15. And 15 plus four more is 19. So we know where 19 centimeters
come from on this shape. Here's the 19 centimeters
are here outlined in blue. So how many more centimeters, what length must this missing side be to get us a total distance around the outside of 24 centimeters. We can do that math,
19 plus one more is 20. And then plus four more
to 24, so one more to 20, and four more, that's
a total of five more. So the missing side length
is five centimeters. This side right here
must be five centimeters in order for the total perimeter to be 24. Okay, let's look at one more. This time it says that the
perimeter of a rectangle is 32 meters. And one side is 11 meters. What is the length of the missing side? So again, we have a missing side. So let's visualize this. Let's draw ourselves a rectangle here. This is what a rectangle looks like. And we know that one
side is 11 centimeters, so one side is 11, or meters, excuse me. And because we know that
this side is 11 meters, we also know that this side is 11 meters, on a rectangle, opposite sides are equal. So those two we know. But what we don't know are
this top and the bottom. Those are the missing sides. So what are those side lengths? Well, again, we know we have
a perimeter of 32 meters, we're told right here
perimeter is 32 meters. So, the question is 11 plus 11 for these two sides, 11 and 11. Plus some mystery number and then plus that same mystery number because these two sides are equal. And that has to give us
a total of equals 32. Because 32 is the total
distance around the outside. So let's add the 11s. 11 plus 11 is 22. So 22 plus how many more gets us to 32? Well, 22 plus 10 more is 32, but if we label one of these sides as 10, then the other one would have to be zero. So they have to be equal,
so how can we equally split those 10 meters between the two sides? Half of 10 is five. So these sides are each five meters. 22, five plus five is ten. 22 plus 10 is 32. So the missing side length is five meters.