How to Find Volumes of Solids of Revolution

Many of the shapes encountered in geometry are solids of revolution, such as spheres, cones and cylinders. The formulas used to calculate their volumes have their basis in calculus. For instance, imagine a semicircle function rotating around the x-axis. Integrate the square of that function and multiply times pi, and you've derived the formula for the volume of a sphere. However, calculus offers much more flexibility for calculating solids of revolution besides just spheres. With it you can derive the volume of any shape whose profile is a function.

Instructions

    • 1

      Find the square of the profile function that traces out the solid of revolution. The general method for finding the volume of a solid of revolution is integrating pi * (f(x))^2. Pi is a constant, so move it outside of the integral and square the profile function. For example, given the function f(x) = x^2/9 + 1, (f(x))^2 = x^4/81 + 2x^2/9 + 1.

    • 2

      Find the anti-derivative of the squared function. For example, g(x) = the anti-derivative of (f(x))^2 = x^5/405 + 2x^3/27 + x + K, where K is some constant. The value of K is unimportant, since it gets eliminated in the next step.

    • 3

      Evaluate the integral within the defined bounds. For example, given the bounds [0, 3], find the difference g(3) - g(0). Plug those values into the function, g(3) = 3/5 + 2 + 3 + K = 28/5 + K, and g(0) = K. Therefore, g(3) - g(0) = 28/5.

    • 4

      Find the volume by multiplying the evaluated integral by pi. The volume of the solid given in the example would be 28 * pi/5, which is 17.6.

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