Solve for X: Rectangle Perimeter 18 with Side Lengths 5x and 10x-8

Rectangle Perimeter with Algebraic Expressions

The shape below is composed of three rectangles.

Calculate x given that the perimeter of rectangle GCHF is 18.

AAABBBCCCGGGFFFHHHDDDEEE10x-83+x5x

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:06 Let's find the value of X.
00:15 In rectangle G F C H, the opposite sides are equal.
00:26 In rectangle A B G C, the opposite sides are equal too.
00:36 The perimeter of a rectangle is the total of all its sides.
00:43 Now, substitute the known values, and solve the equation for X.
01:13 Next, collect like terms together.
01:31 Then, isolate the X to get its value.
01:38 And that's how we solve for X!

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

The shape below is composed of three rectangles.

Calculate x given that the perimeter of rectangle GCHF is 18.

AAABBBCCCGGGFFFHHHDDDEEE10x-83+x5x

2

Step-by-step solution

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

  • Step 1: Write the expression for the perimeter of rectangle GCHF using the given variables.
  • Step 2: Set up the equation by equating the expression to 18.
  • Step 3: Solve the equation for x x .

Let's work through each step:

Step 1: The perimeter of rectangle GCHF is given by the formula:

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

According to the diagram, the dimensions of the rectangle GCHF are:

- GH = 3+x 3 + x (length),

- GF = 5x 5x (width).

Thus, the perimeter formula becomes:

2×(3+x)+2×(5x) 2 \times (3 + x) + 2 \times (5x)

Step 2: Set the equation equal to the given perimeter, 18:

2(3+x)+2(5x)=18 2(3 + x) + 2(5x) = 18

Step 3: Simplify and solve for x x :

Distribute the 2:

6+2x+10x=18 6 + 2x + 10x = 18

Combine like terms:

6+12x=18 6 + 12x = 18

Subtract 6 from both sides:

12x=12 12x = 12

Divide both sides by 12 to isolate x x :

x=1 x = 1

Therefore, the solution to the problem is x=1 x = 1 .

3

Final Answer

1

Key Points to Remember

Essential concepts to master this topic
  • Perimeter Formula: Rectangle perimeter = 2(length) + 2(width)
  • Technique: Substitute dimensions: 2(3+x) + 2(5x) = 18
  • Check: Verify x=1: 2(4) + 2(5) = 8 + 10 = 18 ✓

Common Mistakes

Avoid these frequent errors
  • Adding dimensions instead of using perimeter formula
    Don't just add (3+x) + 5x = 18! This ignores that rectangles have 4 sides, giving wrong answers. Always use the complete perimeter formula: 2(length) + 2(width).

Practice Quiz

Test your knowledge with interactive questions

Look at the rectangle ABCD below.

Side AB is 6 cm long and side BC is 4 cm long.

What is the area of the rectangle?
666444AAABBBCCCDDD

FAQ

Everything you need to know about this question

Why do I multiply each dimension by 2?

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Because a rectangle has 4 sides! You have 2 sides of length (3+x) and 2 sides of width (5x). The perimeter is the total distance around the shape.

How do I identify which sides are length and width?

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Look at the diagram carefully! In rectangle GCHF, the horizontal sides are labeled (3+x) and the vertical sides are labeled 5x.

What if I get a negative value for x?

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Check your work! In geometry problems, dimensions are usually positive. A negative x would mean negative side lengths, which doesn't make physical sense.

Can I solve this by combining like terms first?

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Yes! After setting up 2(3+x)+2(5x)=18 2(3+x) + 2(5x) = 18 , expand to get 6+2x+10x=18 6 + 2x + 10x = 18 , then combine: 6+12x=18 6 + 12x = 18 .

How do I know rectangle GCHF from the diagram?

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Follow the vertices in order: G → C → H → F. These four points form the rectangle we need to find the perimeter for, with given side lengths.

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