A triangular base has vertices \((0,0)\), \((3,0)\), and \((0,3)\). Cross sections perpendicular to the x-axis are squares.
a) Write the cross-sectional area function \(A(x)\).
b) Use \(V=\int A(x)\,\mathrm{d}x\) to find the exact volume.
c) Identify the standard solid formed and verify the same volume with its geometric volume formula.
Hints
- Determine the vertical width of the triangular base at a general x-value.
- That width is a square side length, so how is \(A(x)\) related to it?
- After integrating, compare the dimensions with a familiar pyramid formula.
Solution
1. The slanted boundary through \((0,3)\) and \((3,0)\) is \(y=3-x\), so each square has side length \(s(x)=3-x\) for \(0\le x\le3\).
2. Therefore, \(A(x)=(3-x)^2\).
3. Using cross-sectional integration, \(V=\int_0^3(3-x)^2\,\mathrm{d}x=9\).
4. The linearly shrinking square sections form a square pyramid with base side \(3\) and height \(3\). Its geometric formula gives \(\frac{1}{3}(3^2)(3)=9\), confirming the integral.
Answer
a) \(A(x)=(3-x)^2\)
b) \(V=\int_0^3(3-x)^2\,\mathrm{d}x=9\) cubic units
c) The solid is a square pyramid, and \(\frac{1}{3}(3^2)(3)=9\) cubic units.