# Finding Length of Unknown Sides

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Basics on the topic
**Finding Length of Unknown Sides**

## Content

## Finding the Length of Unknown Sides

To find the length of missing sides on a rectangle;

- Add together known side lengths
- Subtract total from perimeter
- If the shape is a rectangle with unknown opposite sides, divide the answer by two
- If it is a shape with 1 unknown side, the answer is the length
- Check your work by adding all the sides together

Below you will find a worksheet on finding the length of unknown sides.

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Transcript
**Finding Length of Unknown Sides**

It looks like Zuri and Freddie have found something! "We have to build this climbing wall!" "Yes, but there are side lengths missing, Zuri!" Let's help Zuri and Freddie find the length of unknown sides. When finding missing side lengths, we can use the given perimeter to help us. Perimeter is the total distance around the outside of a shape. If no unit of measurement is given, like feet or yards, we use the term units to measure perimeter. You may be asked to find missing side lengths of rectangles, where opposite side lengths are unknown. Before we help Zuri and Freddie, let's see how to find missing side lengths. Here we have a rectangle, with two unknown side lengths. Rectangles have two opposite sides equal in length. In order to find the unknown lengths, first, add together the known side lengths. Three plus three equals six. Second, subtract the total of the known side lengths from the perimeter. Twenty-six minus six equals twenty. Third, since rectangles have opposite sides equal in length, we divide twenty by two, to find find missing side length. Twenty divided by two equals ten. The missing side lengths of the rectangle are ten units each. Finally, check the answer by adding together all sides to make sure it is the same as the perimeter. Three plus three plus ten plus ten equals twenty-six. Let's help Zuri and Freddie find the unknown side lengths on the blueprint now! Here is the frame of the climbing wall. We can see it is a rectangle, with two unknown side lengths. What is the first step? First, add the known side lengths. Six plus six equals twelve. What is the second step? Second, subtract the total of the known side lengths from the perimeter, which is twenty units. Twenty minus twelve equals eight. What is the third step? Since opposite sides of a rectangle are equal in length, we divide eight by two. Eight divided by two equals four. The missing side lengths of the frame are four units each. Finally, we check our work by adding all the sides together to get twenty units. Now let's help Zuri and Freddie find the missing side length of the front of the climbing wall! This time, it is an irregular polygon shape with only one unknown side length. Since only one side length is unknown, we will only need to do the first two steps to solve this one but we will still check our work! What is the first step? First, add the known side lengths. Six plus six plus eight plus two equals twenty-two. What is the second step? Second, subtract the total of the known side lengths from the perimeter, which is twenty-five units. Twenty-five minus twenty-two equals three. Since there is only one unknown side length, this is our answer. The missing side length is three units. Finally, we check our work by adding all the sides together to get twenty-five. While Zuri and Freddie finish building the climbing wall let's review! Remember, you can find unknown side lengths using a given perimeter. First, add together the known side lengths. Second, subtract the total of the known side lengths from the perimeter. Third, if the shape is a rectangle and opposite sides are unknown, divide the answer by two, or if the shape has only one unknown side, this is the unknown length! Finally, check your work by adding all the sides together and check against the perimeter. "Freddie, I can't do this, I'm so scared!" "Me too Zuri... just don't look down!"