# Finding Length of Unknown Sides — Let's Practice!

Rating

Ø 5.0 / 1 ratings
The authors
Team Digital
Finding Length of Unknown Sides — Let's Practice!
CCSS.MATH.CONTENT.3.MD.D.8

## Basics on the topicFinding Length of Unknown Sides — Let's Practice!

Today we are practicing finding lengths of unknown sides with Razzi! This video contains examples to help you further practice and grow confident in this topic.

### TranscriptFinding Length of Unknown Sides — Let's Practice!

Razzi says get these items ready, because today we're going to practice finding length of unknown sides. It's time to begin! Find the missing side lengths of this shape. Pause the video to work on the problem, and press play when you are ready to see the solution! Nine plus nine is eighteen. Subtract eighteen from the perimeter. Twenty-six minus eighteen leaves eight. Both sides are equal in length, so divide eight by two. Eight divided by two equals four. Did you also get four units as your answer? Let's tackle the next problem! Find the missing side length of this shape. Pause the video to work on the problem, and press play when you are ready to see the solution! Six plus ten plus three plus five plus three equals twenty-seven. Subtract twenty-seven from the perimeter. Thirty-two minus twenty-seven leaves five. Did you also get five units for the missing side length? Let's tackle the final problem! Find the missing side length of this shape. Pause the video to work on the problem, and press play when you are ready to see the solution! Four plus six plus six plus three plus two equals twenty-one Subtract twenty-one from the perimeter. Twenty-eight minus twenty-one is seven. Did you also get seven units for the missing side length? Razzi had so much fun practicing with you today! See you next time!

## Finding Length of Unknown Sides — Let's Practice! exercise

Would you like to apply the knowledge you’ve learned? You can review and practice it with the tasks for the video Finding Length of Unknown Sides — Let's Practice!.
• ### What are the missing side lengths?

Hints

Add up all of the given side lengths.

Subtract the answer from the total perimeter.

Solution
1. 6 + 2 + 2 + 3 + 5 = 18; 22 - 18 = 4. The missing length is 4 units.
2. 2 + 8 + 7 + 2 + 6 = 25; 30 - 25 = 5. The missing length is 5 units.
3. 3 + 2 + 4 + 6 + 2 + 1 + 10 = 28; 34 - 28 = 6. The missing length is 6 units.
4. 10 + 2 + 6 + 3 + 4 = 25; 30 - 25 = 5. The missing length is 5 units.
• ### Can you find the missing lengths?

Hints

Remember, opposite sides are equal in length on a rectangle.

Add the two given side lengths and subtract that from the perimeter.

Once you have done that, divide what is left by 2 to find one of the missing side lengths.

Solution

Grassy area

• 6 + 6 = 12
• 32 - 12 = 20
• 20 $\div$ 2 = 10
• Each missing side length is 10 m

Bark area

• 5 + 5 = 10
• 18 - 10 = 8
• 8 $\div$ 2 = 4
• Each missing side length is 4 m.

• ### What are the missing side lengths?

Hints

Don't forget, the perimeter for all of these shapes is 22 units.

Remember, opposite sides are equal on a rectangle.

If you are finding the missing side of a rectangle:

• Subtract this from the total perimeter.
• Divide the answer by two.

If you are finding the missing side of a different shape:

• Subtract this from the total perimeter.
• What is left is the missing side length.

Solution

For the first shape:

• 7 + 7 = 14
• 22 - 14 = 8
• 8 $\div$ 2 = 4
For the second shape:
• 6 + 4 + 5 + 1 + 1 = 17
• 22 - 17 = 5
For the third shape:
• 2 + 2 = 4
• 22 - 4 = 18
• 18 $\div$ 2 = 9
For the fourth shape:
• 2 + 4 + 3 + 2 + 5 = 16
• 22 - 16 = 6

• ### What are the missing side lengths of the floor plan?

Hints

Both of the rooms are rectangles. What do you know about the side lengths of a rectangle?

To find the missing side length of room A, first add 4 + 4 and then subtract this from 24.

Next, divide the answer by 2.

Can you follow these steps to find the missing side lengths of room B?

Here is the floor plan partially filled in. To find the remaining missing side lengths, add 5 + 5 then subtract it from 24 before dividing the answer by 2.

Solution

Room A

• 4 + 4 = 8
• 24 - 8 = 16
• 16 $\div$ 2 = 8
• The missing length is 8 units.
Room B
• It is a rectangle, so even though there is only one 5 given, we know that the opposite side is also 5.
• 5 + 5 = 10
• 24 - 10 = 14
• 14 $\div$ 2 = 7
• The missing lengths are 7 units each.

• ### Find the missing side length.

Hints

Add up all of the given side lengths first.

Subtract this from the total perimeter.

6 + 2 + 2 + 4 + 8 = 22

28 - 22 = ?

Solution

The missing length was 6 units.

• 6 + 2 + 2 + 4 + 8 = 22
• 28 - 22 = 6
• ### Find the missing values.

Hints

You could start by finding the missing side lengths in shape A or C.

Once you have found the missing side length in shape C, you will be able to find the perimeter of shape B.

Remember, a rectangle's opposite sides are equal.

Solution

Shape A

• We are given a length of 4 and we know the other part of that side is 5, so we add 4 + 5 to make 9.
• This means the opposite side is also 9.
• We can then add 9 + 9 = 18 and subtract 18 from the perimeter, 26.
• 26 - 18 = 8
• 8 $\div$ 2 = 4
• The other missing side lengths are 4 units.

Shape C

• 8 + 6 + 3 + 4 + 4 = 25
• 28 - 25 = 3

Shape B

• Now we know the missing side length of shape C is 3 units, we can add 2 + 3 + 3 = 8.
• We then know the opposite side is 8 units.
• We can then add 8 + 5 + 8 + 5 = 26.
• The perimeter of shape B is 26 units.