A square playground is redesigned as a rectangle by increasing one side by \(6\,\text{ft}\) and decreasing the other side by \(4\,\text{ft}\). The new area is \(16\,\text{ft}^2\) greater than the original area.
a) Find the original side length of the square.
b) Someone claims, “If one side is increased by the same amount that the other side is decreased, the area always stays the same.” Use the side length from part a) and changes of \(5\,\text{ft}\) to show that the claim is false.
Hints
- Write area expressions for the square and the redesigned rectangle.
- The quadratic terms should cancel when you compare the areas.
- For part b), calculate both areas using the value from part a).
- A single counterexample is enough to disprove an “always” claim.
Solution
1. Let \(s\) feet be the square's side length. Its area is \(s^2\).
2. The redesigned rectangle has dimensions \(s + 6\) and \(s - 4\), so its area is \((s + 6)(s - 4) = s^2 + 2s - 24\).
3. The new area is \(16\,\text{ft}^2\) greater, so \(s^2 + 2s - 24 = s^2 + 16\).
4. Subtract \(s^2\) and solve: \(2s - 24 = 16\), so \(2s = 40\) and \(s = 20\).
5. For part b), the original area is \(20 \cdot 20 = 400\,\text{ft}^2\).
6. Changing the sides by \(5\,\text{ft}\) gives dimensions \(25\,\text{ft}\) and \(15\,\text{ft}\), with area \(25 \cdot 15 = 375\,\text{ft}^2\).
7. Since \(375 \neq 400\), the claim is false.
Answer
a) The original side length was \(20\,\text{ft}\).
b) The changed dimensions are \(25\,\text{ft}\) by \(15\,\text{ft}\), with area \(375\,\text{ft}^2\), not \(400\,\text{ft}^2\). Therefore, the claim is false.