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Finding perimeter when a side length is missing

CCSS.Math:

Video transcript

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 links 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 5 centimeters so we have 5 centimeters plus moving down the side here we have another 3 centimeters and then going across next distance is 4 centimeters after that we go down this side right here which is another 4 centimeters and as we keep going across the bottom we have another 9 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 got to know the entire distance she walked so what we can do is we can look over here and say this length right here is 3 centimeters well then that same length over here is also 3 centimeters and where that 3 centimeters left off this 4 centimeters this length picked up so from here to here is another 4 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 because that's the same as seven nine plus one is ten and ten plus six is sixteen eight plus eight is another sixteen so we have a 16 and a 16 16 let's see six 1s 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 were 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