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## Perimeter

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# Finding perimeter when a side length is missing

CCSS Math: 3.MD.D.8

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## Video transcript

- [Voiceover] What is the
perimeter of the figure below? So down here, we have this figure, and we are asked to find the perimeter of this figure. Perimeter is the distance all the way around the outside of a shape. So in this case, if I were to walk around the outside, or maybe a human's too big, if an awesome little ant was to walk all the way around the
outside of this figure, how far she walked would be the perimeter. So to find that, what we can do is figure out how long is this side, and then combine it with
the length of this side, and combine it with the
length of this side, and once we combine all
of the side lengths, we'll have the perimeter, or the distance around
the outside of the figure. So let's start, we can start up here. We can see that this side length is five centimeters. So we have five centimeters, plus, moving down the side here, we have another three centimeters, and then going across, next distance is four centimeters. After that, we go down
this side right here, which is another four centimeters, and as we keep going, across the bottom, we have another nine centimeters. And then, we head up this side, and uh oh, we don't have a label. We don't know how far this is. But to find perimeter, we need the distance
around the entire outside. If this little ant, she walked the whole way, we've gotta know the
entire distance she walked. So what we can do is, we can look over here and say, this length right here
is three centimeters. Well then, that same length over here is also three centimeters. And where that three centimeters left off, this four centimeters, this length, picked up. So from here to here is another four centimeters. So if we have four centimeters plus another three centimeters, that's a total of seven more centimeters. So now we know the entire, all of the lengths, around the outside. So if we combine them or add them, we'll know the perimeter. So we can start, five plus three is eight. Four plus four is another eight. Nine plus seven, let's
see, instead of seven, I could do nine plus one and six, 'cause that's the same as seven. Nine plus one is 10, and 10 plus six is 16. Eight plus eight is another 16, so we have a 16 and a 16. 16, let's see, six ones plus six more ones is 12 ones, and one 10 plus another 10, or 10 plus 10 is 20. So another way we could add this is 20 plus 12, which is a total of 32. And we're talking about centimeters here, so 32 centimeters. The distance all the way around the outside of this figure, or the perimeter of the figure, is 32 centimeters.