Rectangle Perimeter Problem: Solving with Dimensions (X+5) and (X-1)

Quadratic Applications with Area Constraints

Look at the following rectangle:

AAABBBCCCDDDX+5X-17

The area of the rectangle is 7.

What is the perimeter of the rectangle?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Calculate the perimeter of the rectangle
00:03 Use the formula for calculating the area of a rectangle (side multiplied by side)
00:14 Substitute appropriate values according to the given data and solve for X
00:30 Open parentheses properly, multiply each factor by each factor
00:50 Arrange the equation so that one side equals 0
00:58 Collect like terms
01:07 Use trinomial method and pay attention to coefficients
01:11 We want to find 2 numbers whose sum equals 4(B)
01:21 And their product equals (-12)(C)
01:24 These are the appropriate numbers, let's put them in parentheses
01:33 Find the solutions that make each factor equal to zero
01:40 X must be positive since it's a value of a side
01:44 Therefore this solution is the solution for X
01:54 This is the value of X, let's substitute to find the side length
02:06 Opposite sides are equal in a rectangle
02:26 The perimeter of the rectangle equals the sum of its sides
02:36 Insert the appropriate values into the formula and solve for the perimeter
02:47 This is the solution

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Look at the following rectangle:

AAABBBCCCDDDX+5X-17

The area of the rectangle is 7.

What is the perimeter of the rectangle?

2

Step-by-step solution

To solve this problem, we'll need to find the value of X X and then use this value to find the perimeter of the rectangle. Follow these detailed steps:

  • Step 1: Set up the equation for the area.
    Given the area of the rectangle is 7, and the sides are (X+5) (X + 5) and (X1) (X - 1) , the equation becomes:

(X+5)(X1)=7(X + 5)(X - 1) = 7
Expanding the left side, we have: X2+5XX5=7 X^2 + 5X - X - 5 = 7 X2+4X5=7 X^2 + 4X - 5 = 7

  • Step 2: Simplify and solve the quadratic equation.
    By moving all terms to one side, we have:

X2+4X57=0 X^2 + 4X - 5 - 7 = 0 X2+4X12=0 X^2 + 4X - 12 = 0

  • Step 3: Factor the quadratic equation.
    We factor the equation as:

(X+6)(X2)=0 (X + 6)(X - 2) = 0

  • Step 4: Solve for X X using the zero-product property.
    The solutions for X X are:

X+6=0X=6 X + 6 = 0 \Rightarrow X = -6 X2=0X=2 X - 2 = 0 \Rightarrow X = 2

  • Step 5: Use the positive X X value to find the perimeter.
    Since dimensions cannot be negative, X=2 X = 2 . Thus, the rectangle’s dimensions become:

Length=X+5=2+5=7 \text{Length} = X + 5 = 2 + 5 = 7
Width=X1=21=1 \text{Width} = X - 1 = 2 - 1 = 1

  • Step 6: Calculate the perimeter using the dimensions.

P=2×(Length+Width)=2×(7+1)=2×8=16 P = 2 \times (\text{Length} + \text{Width}) = 2 \times (7 + 1) = 2 \times 8 = 16

Therefore, the perimeter of the rectangle is 16.

3

Final Answer

16

Key Points to Remember

Essential concepts to master this topic
  • Setup: Area formula gives (X+5)(X1)=7 (X+5)(X-1) = 7 equation
  • Technique: Expand to X2+4X12=0 X^2 + 4X - 12 = 0 then factor
  • Check: Dimensions must be positive: X = 2 gives 7×1 = 7 ✓

Common Mistakes

Avoid these frequent errors
  • Using negative value for X
    Don't accept X = -6 because it gives negative dimensions = impossible rectangle! Negative lengths don't exist in real geometry. Always check that your X value makes both dimensions positive.

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 can't I use X = -6 as my answer?

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When X = -6, one dimension becomes X1=61=7 X - 1 = -6 - 1 = -7 . Since rectangles can't have negative side lengths, we must use X = 2 instead.

How do I know which factoring method to use?

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For X2+4X12=0 X^2 + 4X - 12 = 0 , look for two numbers that multiply to -12 and add to 4. That's 6 and -2, giving us (X+6)(X2)=0 (X+6)(X-2) = 0 .

Can I solve this without expanding the equation?

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You could try, but expanding (X+5)(X1)=7 (X+5)(X-1) = 7 to standard form makes factoring much easier. Always move to standard form for quadratic equations!

What if I get the area calculation wrong?

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Double-check: Length × Width = 7×1=7 7 \times 1 = 7 ✓. If your dimensions don't give the correct area, go back and re-solve the quadratic equation.

How do I calculate perimeter from the dimensions?

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Use the formula: Perimeter = 2(Length + Width). With dimensions 7 and 1: P=2(7+1)=2(8)=16 P = 2(7 + 1) = 2(8) = 16 .

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