The region under y = 0.5x from x = 0 to x = 3 is rotated about the x-axis. Find the volume V of the solid (a cone).
Find V
Show hints
- A slice at x is a disc of radius y = 0.5x.
- V = π∫₀3 (0.5x)² dx.
- = π·0.25·x³/3.
Show the answer and the step-by-step solution
Volumes of revolution: rotating the region under y = f(x) from x = a to x = b about the x-axis makes a solid whose cross-section at x is a disc of radius f(x). Adding the discs gives V = π∫ₐᵇ f(x)² dx. When the region lies between an outer curve R(x) and an inner curve r(x), each slice is a washer: V = π∫ₐᵇ (R(x)² − r(x)²) dx. Square the radii before you subtract them.
Disc method
Integrate
Answer
Check: a cone of radius 1.5 and height 3 has (1/3)πr²h = 7.069.