Calculate Rectangle Area: Finding (x-7)(x+8) Using Algebraic Expressions

Rectangle Area with Binomial Multiplication

What is the area of a rectangle with sides of x-7 and x+8?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:06 Let's find the area of the rectangle.
00:09 To do this, we multiply the length by the width. Remember, it's side times side.
00:15 Next, expand the brackets carefully. Make sure each term multiplies every other term.
00:35 Now, let's group similar terms and do the calculations.
00:41 Great work! That's how we solve the problem.

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

What is the area of a rectangle with sides of x-7 and x+8?

2

Step-by-step solution

To tackle this problem, we must compute the area of a rectangle given its sides are expressed as x7 x-7 and x+8 x+8 .

Step 1: Start by recognizing the formula used to calculate the area of a rectangle: the product of its length and width.

Step 2: Substitute the given expressions for length and width into this formula:

Area=(x7)(x+8) \text{Area} = (x-7)(x+8)

Step 3: Multiply the binomial expressions using the distributive property, often referred to as the FOIL method (First, Outer, Inner, Last).

  • First terms: x×x=x2 x \times x = x^2
  • Outer terms: x×8=8x x \times 8 = 8x
  • Inner terms: 7×x=7x -7 \times x = -7x
  • Last terms: 7×8=56 -7 \times 8 = -56

Step 4: Combine all these products into a single polynomial expression:

x2+8x7x56 x^2 + 8x - 7x - 56

Step 5: Simplify by combining like terms:

x2+(8x7x)56=x2+x56 x^2 + (8x - 7x) - 56 = x^2 + x - 56

Therefore, the area of the rectangle is given by the polynomial expression: x2+x56 x^2 + x - 56 .

3

Final Answer

x2+x56 x^2+x-56

Key Points to Remember

Essential concepts to master this topic
  • Formula: Rectangle area equals length times width for algebraic expressions
  • FOIL Method: (x7)(x+8)=x2+8x7x56 (x-7)(x+8) = x^2 + 8x - 7x - 56
  • Verification: Combine like terms: 8x - 7x = x, final answer x2+x56 x^2 + x - 56

Common Mistakes

Avoid these frequent errors
  • Forgetting to multiply all terms when expanding binomials
    Don't just multiply x × x and -7 × 8 = wrong partial result! This skips the outer and inner terms, missing crucial parts of the expansion. Always use FOIL method: multiply First, Outer, Inner, and Last terms completely.

Practice Quiz

Test your knowledge with interactive questions

\( x^2+6x+9=0 \)

What is the value of X?

FAQ

Everything you need to know about this question

Why can't I just add (x-7) + (x+8) to find the area?

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Area requires multiplication, not addition! Adding gives you the perimeter formula. For rectangles, you must multiply length times width: (x7)×(x+8) (x-7) \times (x+8) .

What does FOIL stand for and why do I need it?

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FOIL means First, Outer, Inner, Last - the four products when multiplying binomials. It ensures you don't miss any terms when expanding (x7)(x+8) (x-7)(x+8) .

How do I know which terms to combine?

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Look for like terms - terms with the same variable and exponent. In this problem, combine 8x 8x and 7x -7x because they're both x terms: 8x7x=x 8x - 7x = x .

Can the area be negative if x is small?

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Yes! If x < 7, then (x-7) is negative. Since areas represent real measurements, this tells us the rectangle only exists when both dimensions are positive.

How can I check if my expanded form is correct?

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Pick a simple value for x (like x = 10) and calculate both (x7)(x+8) (x-7)(x+8) and your expanded form. Both should give the same result: (3)(18)=54 (3)(18) = 54 and 100+1056=54 100 + 10 - 56 = 54

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