Math Problem Statement

Question 4

Exam-style question

A rectangular garden measuring 15×18 (metres) needs to have a path made around it.

The path has the same width on each side of the garden.

This is shown in the diagram:

The builder only has enough material to build a path of total area of 150 m2.

Find the width of the path (in m) if he uses up all the materials he has.

Enter your answer as a decimal, correct to three significant figures. Do not include the unit with your answer.

Solution

To solve this problem, we need to set up an equation based on the given information:

  • The original rectangular garden has dimensions 15 m by 18 m.
  • A path of equal width is built around the garden, and the total area of the path is 150 m².
  • We are asked to find the width of the path.

Step 1: Define the variable

Let the width of the path be xx meters.

Step 2: Set up the dimensions of the new, larger rectangle

The path surrounds the garden, so it adds xx meters to each side of the garden. The new dimensions of the rectangle, including the path, will be:

  • Length: 18+2x18 + 2x (since the path adds xx meters on both sides of the length).
  • Width: 15+2x15 + 2x (since the path adds xx meters on both sides of the width).

Step 3: Calculate the area of the new, larger rectangle

The area of the larger rectangle is: Area of larger rectangle=(18+2x)(15+2x)\text{Area of larger rectangle} = (18 + 2x)(15 + 2x)

Step 4: Subtract the area of the original garden

The area of the garden without the path is: Area of the garden=18×15=270m2\text{Area of the garden} = 18 \times 15 = 270 \, \text{m}^2 The area of the path is the difference between the area of the larger rectangle and the area of the original garden: Area of the path=(18+2x)(15+2x)270\text{Area of the path} = (18 + 2x)(15 + 2x) - 270 We are told that the area of the path is 150 m²: (18+2x)(15+2x)270=150(18 + 2x)(15 + 2x) - 270 = 150

Step 5: Solve the equation

Expand the left-hand side: (18+2x)(15+2x)=18×15+18×2x+15×2x+2x×2x(18 + 2x)(15 + 2x) = 18 \times 15 + 18 \times 2x + 15 \times 2x + 2x \times 2x =270+36x+30x+4x2= 270 + 36x + 30x + 4x^2 =270+66x+4x2= 270 + 66x + 4x^2 Substitute this into the equation: 270+66x+4x2270=150270 + 66x + 4x^2 - 270 = 150 Simplifying: 66x+4x2=15066x + 4x^2 = 150 Rearrange the equation: 4x2+66x150=04x^2 + 66x - 150 = 0

Step 6: Solve the quadratic equation

To solve 4x2+66x150=04x^2 + 66x - 150 = 0, we can use the quadratic formula: x=b±b24ac2ax = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} For the equation 4x2+66x150=04x^2 + 66x - 150 = 0, we have a=4a = 4, b=66b = 66, and c=150c = -150.

Substitute these values into the quadratic formula: x=66±6624(4)(150)2(4)x = \frac{-66 \pm \sqrt{66^2 - 4(4)(-150)}}{2(4)} x=66±4356+24008x = \frac{-66 \pm \sqrt{4356 + 2400}}{8} x=66±67568x = \frac{-66 \pm \sqrt{6756}}{8} x=66±82.28x = \frac{-66 \pm 82.2}{8}

Now solve for the two possible values for xx: x=66+82.28orx=6682.28x = \frac{-66 + 82.2}{8} \quad \text{or} \quad x = \frac{-66 - 82.2}{8} x=16.28=2.025orx=148.28=18.525x = \frac{16.2}{8} = 2.025 \quad \text{or} \quad x = \frac{-148.2}{8} = -18.525

Since the width of the path cannot be negative, we discard x=18.525x = -18.525 and keep x=2.025x = 2.025.

Final Answer:

The width of the path is 2.03 meters (rounded to three significant figures).


Would you like a detailed breakdown of any step? Here are some follow-up questions:

  1. How does the quadratic formula work in this context?
  2. What would happen if the area of the path was larger or smaller?
  3. How could you model this problem with a different shape, like a circular garden?
  4. How would the problem change if the path had varying widths along different sides?
  5. Can you explain how to solve quadratic equations in general?

Tip: Always check for physical meaning in your solutions. Negative values may be mathematically correct but don't always fit real-world contexts!

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Math Problem Analysis

Mathematical Concepts

Algebra
Quadratic Equations
Area of Rectangles

Formulas

Area of rectangle = Length × Width
Quadratic equation formula ax^2 + bx + c = 0
Quadratic formula: x = (-b ± √(b² - 4ac)) / 2a

Theorems

Quadratic formula

Suitable Grade Level

Grades 8-10