Calculate x in Rectangular Area Equation: 2(x-4) by 2x+8 Yields 132

Quadratic Equations with Rectangle Area

The area of the rectangle below is 132.

Calculate x.

2(x-4)2(x-4)2(x-4)2x+82x+82x+8

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find X
00:03 Use the formula for calculating rectangle area (side times side)
00:09 Substitute the side values according to the given data
00:19 Substitute the area size according to the given data and solve for X
00:30 Open parentheses properly
00:38 Use the shortened multiplication formulas to expand the parentheses
00:59 Calculate 2 squared and 8 squared
01:06 Isolate X
01:23 Take the square root to find the possible solutions for X
01:29 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

The area of the rectangle below is 132.

Calculate x.

2(x-4)2(x-4)2(x-4)2x+82x+82x+8

2

Step-by-step solution

To solve the problem, let's proceed with the following steps:

  • Step 1: Identify the area formula
  • Step 2: Substitute side expressions
  • Step 3: Simplify the equation and solve

Let's begin:
Step 1: The area of the rectangle is given by the formula:
Area=Length×Width \text{Area} = \text{Length} \times \text{Width} For this rectangle, the length and width are given as 2x+8 2x + 8 and 2(x4) 2(x-4) , respectively. Thus, we have the equation for the area:

(2x+8)×(2(x4))=132 (2x + 8) \times (2(x - 4)) = 132

Step 2: Substitute the side expressions and set up the equation:

(2x+8)×(2x8)=132 (2x + 8) \times (2x - 8) = 132

Step 3: Expand and solve:

  • First, we expand the expression using multiplication:
  • The side expression simplifies to a difference of squares:
(2x+8)(2x8)=(2x)282 (2x + 8)(2x - 8) = (2x)^2 - 8^2 =4x264 = 4x^2 - 64

The area equation is:

4x264=132 4x^2 - 64 = 132

Step 4: Simplify and solve the quadratic equation:

  • Add 64 64 to both sides to isolate the quadratic term:
  • 4x2=196 4x^2 = 196
  • Divide by 4 4 :
  • x2=49 x^2 = 49
  • Take the square root:
  • x=±49 x = \pm \sqrt{49} x=±7 x = \pm 7

Therefore, the solutions for x x are x=7 x = 7 and x=7 x = -7 . Since area is scalar, both positive and negative solutions are valid for x x .

Thus, the correct value for x x is ±7 \pm 7 .

3

Final Answer

±7 \pm7

Key Points to Remember

Essential concepts to master this topic
  • Area Formula: Rectangle area equals length times width
  • Technique: Use difference of squares: (2x+8)(2x8)=4x264 (2x+8)(2x-8) = 4x^2-64
  • Check: Substitute x=±7 x = \pm 7 back: 4(49)64=132 4(49) - 64 = 132

Common Mistakes

Avoid these frequent errors
  • Incorrectly expanding the product of expressions
    Don't expand (2x+8)×2(x4) (2x+8) \times 2(x-4) as 4x232 4x^2 - 32 = wrong equation! This ignores the distributive property completely. Always first simplify 2(x4)=2x8 2(x-4) = 2x-8 , then use difference of squares pattern.

Practice Quiz

Test your knowledge with interactive questions

Solve:

\( (2+x)(2-x)=0 \)

FAQ

Everything you need to know about this question

Why do I get two answers for x?

+

When you take the square root of both sides, you get both positive and negative solutions. Since x2=49 x^2 = 49 , both x=7 x = 7 and x=7 x = -7 work because 72=(7)2=49 7^2 = (-7)^2 = 49 !

How can x be negative if it's measuring a rectangle?

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In pure geometry, lengths are positive. But in algebra problems, we solve for the variable mathematically first. Both solutions ±7 \pm 7 satisfy the equation, even if only positive values make physical sense.

What's the difference of squares pattern?

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The pattern is (a+b)(ab)=a2b2 (a+b)(a-b) = a^2 - b^2 . Here, (2x+8)(2x8) (2x+8)(2x-8) becomes (2x)282=4x264 (2x)^2 - 8^2 = 4x^2 - 64 . This shortcut saves time compared to full expansion!

How do I check if my answer is correct?

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Substitute both values back into the original area equation. For x=7 x = 7 : length = 2(7)+8=22 2(7)+8 = 22 , width = 2(74)=6 2(7-4) = 6 , so area = 22×6=132 22 \times 6 = 132

Can I solve this without using difference of squares?

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Yes! You can expand everything manually: (2x+8)(2x8)=4x216x+16x64=4x264 (2x+8)(2x-8) = 4x^2 - 16x + 16x - 64 = 4x^2 - 64 . But recognizing the pattern makes it much faster!

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