Calculate the Area of a Rectangle: Visual Geometry Problem

Rectangle Area with Polynomial Expressions

Look at the rectangle in the figure.

What is its area?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the area of the rectangle
00:03 We'll use the formula for calculating rectangle area (side times side)
00:07 Pay attention to parentheses
00:23 Collect terms
00:35 Open parentheses properly, multiply each term by each term
01:17 Calculate the products
01:34 Collect terms
01:42 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

Look at the rectangle in the figure.

What is its area?

2

Step-by-step solution

We know that the area of a rectangle is equal to its length multiplied by its width.

We begin by writing an equation with the available data.

(4x+x2)×(3x+8+5x) (4x+x^2)\times(3x+8+5x)

Next we use the distributive property to solve the equation.

(4x×3x)+(4x×8)+(4x×5x)+(x2×3x)+(x2×8)+(x2×5x)= (4x\times3x)+(4x\times8)+(4x\times5x)+(x^2\times3x)+(x^2\times8)+(x^2\times5x)=

We then solve each of the exercises within the parentheses:

12x2+32x+20x2+3x3+16x2+5x3= 12x^2+32x+20x^2+3x^3+16x^2+5x^3=

Finally we add up all the coefficients of X squared and all the coefficients of X cubed and we obtain the following:

48x2+8x3+32x 48x^2+8x^3+32x

3

Final Answer

8x3+28x2+44x 8x^3+28x^2+44x

Key Points to Remember

Essential concepts to master this topic
  • Formula: Rectangle area equals length times width (A = l × w)
  • Technique: Use distributive property: (4x+x2)×(8x+8)=32x2+32x+8x3+8x2 (4x+x^2) \times (8x+8) = 32x^2+32x+8x^3+8x^2
  • Check: Combine like terms and verify highest degree term matches answer choices ✓

Common Mistakes

Avoid these frequent errors
  • Adding dimensions instead of multiplying
    Don't add length + width = (4x+x2)+(8x+8)=x2+12x+8 (4x+x^2) + (8x+8) = x^2+12x+8 ! This gives perimeter, not area. Always multiply length × width for rectangle area.

Practice Quiz

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\( (x+y)(x-y)= \)

FAQ

Everything you need to know about this question

How do I multiply expressions with multiple terms?

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Use the distributive property! Multiply each term in the first expression by each term in the second expression. For (4x+x2)×(8x+8) (4x+x^2) \times (8x+8) , you get 4 products: 4x×8x, 4x×8, x²×8x, x²×8.

Why does my answer have x³ terms when the choices show different powers?

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That's normal! When you multiply x2×x x^2 \times x , you get x3 x^3 . The final answer should be arranged by decreasing powers: cubic terms first, then quadratic, then linear.

How do I combine like terms after multiplying?

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Group terms with the same variable power together. For example: 12x2+20x2+16x2=48x2 12x^2 + 20x^2 + 16x^2 = 48x^2 . Add the coefficients (numbers) but keep the variable part the same.

What if I get confused about which dimension is length vs width?

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It doesn't matter! Rectangle area is commutative, meaning length × width = width × length. Just make sure you multiply the two expressions representing the sides.

How can I check if my final answer is correct?

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Look at the degree (highest power) and leading coefficient. Also, try substituting a simple value like x=1 into both your answer and the original expression to see if they match.

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