Math Problem Statement

The shaded area is 75 m². Work out the area of the smaller rectangle.

Solution

We are given the problem where the total shaded area is 75m275 \, \text{m}^2, and we need to find the area of the smaller rectangle. Here is the approach:

Step-by-step solution:

  1. Outer rectangle dimensions:
    The dimensions of the outer green rectangle are:

    • Length = x+4mx + 4 \, \text{m}
    • Width = 2x+3m2x + 3 \, \text{m}

    So, the area of the outer rectangle is: Aouter=(x+4)(2x+3)A_{\text{outer}} = (x + 4)(2x + 3)

  2. Inner (smaller) rectangle dimensions:
    The dimensions of the smaller rectangle (which is white in the image) are:

    • Length = x+2mx + 2 \, \text{m}
    • Width = x+1mx + 1 \, \text{m}

    So, the area of the inner rectangle is: Ainner=(x+2)(x+1)A_{\text{inner}} = (x + 2)(x + 1)

  3. Shaded area:
    The shaded area is the difference between the outer rectangle and the inner rectangle, and we know it equals 75m275 \, \text{m}^2: Ashaded=AouterAinner=75A_{\text{shaded}} = A_{\text{outer}} - A_{\text{inner}} = 75

  4. Set up the equation:
    Expanding both expressions:

    • Aouter=(x+4)(2x+3)=2x2+3x+8x+12=2x2+11x+12A_{\text{outer}} = (x + 4)(2x + 3) = 2x^2 + 3x + 8x + 12 = 2x^2 + 11x + 12
    • Ainner=(x+2)(x+1)=x2+x+2x+2=x2+3x+2A_{\text{inner}} = (x + 2)(x + 1) = x^2 + x + 2x + 2 = x^2 + 3x + 2

    Subtracting these: Ashaded=(2x2+11x+12)(x2+3x+2)=x2+8x+10A_{\text{shaded}} = (2x^2 + 11x + 12) - (x^2 + 3x + 2) = x^2 + 8x + 10

    Setting it equal to 75: x2+8x+10=75x^2 + 8x + 10 = 75

  5. Solve the quadratic equation:
    Rearrange the equation: x2+8x65=0x^2 + 8x - 65 = 0

    Solve this quadratic equation using the quadratic formula: x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} where a=1a = 1, b=8b = 8, and c=65c = -65.

    Substituting the values: x=8±824(1)(65)2(1)=8±64+2602=8±3242=8±182x = \frac{-8 \pm \sqrt{8^2 - 4(1)(-65)}}{2(1)} = \frac{-8 \pm \sqrt{64 + 260}}{2} = \frac{-8 \pm \sqrt{324}}{2} = \frac{-8 \pm 18}{2}

    So, x=8+182=102=5x = \frac{-8 + 18}{2} = \frac{10}{2} = 5.

  6. Area of the smaller rectangle:
    Now that x=5x = 5, we substitute this into the expression for the area of the smaller rectangle: Ainner=(x+2)(x+1)=(5+2)(5+1)=7×6=42m2A_{\text{inner}} = (x + 2)(x + 1) = (5 + 2)(5 + 1) = 7 \times 6 = 42 \, \text{m}^2

Final Answer:

The area of the smaller rectangle is 42m242 \, \text{m}^2.


Do you have any questions or would you like further details?

Here are some related questions you might consider:

  1. How would the problem change if the shaded area was different?
  2. What happens if we change the dimensions of the inner rectangle?
  3. Can we solve for xx if the outer rectangle has different dimensions?
  4. How does the quadratic formula handle cases where there are no real roots?
  5. Can the shaded area ever be negative in a similar setup?

Tip: Always check that your calculated areas make sense in the context of the problem to avoid logical errors!

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Math Problem Analysis

Mathematical Concepts

Geometry
Algebra
Quadratic Equations

Formulas

Area of a rectangle: A = length × width
Quadratic equation: ax^2 + bx + c = 0

Theorems

Quadratic Formula

Suitable Grade Level

Grades 8-10