Rectangle Perimeter Calculation: Finding the Distance Around a 4x5 Shape

Rectangle Perimeter with Pythagorean Theorem

Calculate the perimeter of following rectangle:

AAABBBCCCDDD45

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the perimeter of the rectangle
00:03 Use the Pythagorean theorem in triangle ACD to find AD
00:15 Substitute appropriate values according to the given data and solve for AD
00:29 Isolate AD
00:50 This is the value of side AD
00:54 Opposite sides are equal in a rectangle
01:06 The perimeter of the rectangle equals the sum of its sides
01:16 Substitute appropriate values and solve for the perimeter
01:38 And this is the solution to the problem

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Calculate the perimeter of following rectangle:

AAABBBCCCDDD45

2

Step-by-step solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given information.
  • Step 2: Apply the Pythagorean theorem to find the missing side.
  • Step 3: Use the perimeter formula for rectangles.

Now, let's work through each step:

Step 1: The given information includes the base (AB=4 AB = 4 ) and the diagonal (AC=5 AC = 5 ).

Step 2: Apply the Pythagorean theorem. The diagonal acts as the hypotenuse of a right triangle with sides being the rectangle's base and height. Using the theorem:

(AB)2+(AD)2=(AC)2 (AB)^2 + (AD)^2 = (AC)^2

42+(AD)2=52 4^2 + (AD)^2 = 5^2

16+(AD)2=25 16 + (AD)^2 = 25

Solving for (AD)2 (AD)^2 :

(AD)2=2516=9 (AD)^2 = 25 - 16 = 9

Thus, AD=9=3 AD = \sqrt{9} = 3 .

Step 3: Calculate the perimeter using the formula for the perimeter of a rectangle:

P=2×(length+width) P = 2 \times (\text{length} + \text{width}) .

Inserting the values AB=4 AB = 4 and AD=3 AD = 3 :

P=2×(4+3)=2×7=14 P = 2 \times (4 + 3) = 2 \times 7 = 14 .

Therefore, the solution to the problem is 14 14 .

3

Final Answer

14

Key Points to Remember

Essential concepts to master this topic
  • Formula: Perimeter equals 2 times length plus width
  • Technique: Use Pythagorean theorem: 42+h2=52 4^2 + h^2 = 5^2 gives height = 3
  • Check: Verify diagonal calculation: 42+32=25=5 \sqrt{4^2 + 3^2} = \sqrt{25} = 5

Common Mistakes

Avoid these frequent errors
  • Adding all given measurements together
    Don't add base + diagonal = 4 + 5 = 9 and call it perimeter! The diagonal is NOT a side of the rectangle. Always find the missing height using Pythagorean theorem, then use P = 2(length + width).

Practice Quiz

Test your knowledge with interactive questions

Look at the rectangle below.

Side DC has a length of 1.5 cm and side AD has a length of 9.5 cm.

What is the perimeter of the rectangle?

1.51.51.5AAABBBCCCDDD9.5

FAQ

Everything you need to know about this question

Why can't I just use the diagonal as one of the sides?

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The diagonal is not part of the rectangle's perimeter! The perimeter only includes the four outer edges. The diagonal cuts through the rectangle, so it's used to find the missing side length.

How do I know which sides are length and width?

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It doesn't matter which you call length or width! Since the perimeter formula is P=2(l+w) P = 2(l + w) , you'll get the same answer whether you use 2(4 + 3) or 2(3 + 4).

What if I can't remember the Pythagorean theorem?

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Remember: a2+b2=c2 a^2 + b^2 = c^2 where c is always the longest side (hypotenuse). In rectangles, the diagonal is always the hypotenuse!

Can I solve this without the Pythagorean theorem?

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No, because you need both dimensions of the rectangle to find perimeter. Since only one side and the diagonal are given, the Pythagorean theorem is the only way to find the missing side.

What if my height calculation gives me a decimal?

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That's fine! Just make sure to calculate carefully and double-check by substituting back into the Pythagorean theorem. Some rectangle problems do have decimal dimensions.

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