Calculating Volume of Rotational Solids Using Integration

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SUMMARY

The discussion focuses on calculating the volume of a solid formed by rotating the region bounded by the curves y=x² and y=2x in the first quadrant about the x-axis. The incorrect integral used was ∫ from 0 to 2 (2x - x²)² dx, which led to an erroneous volume calculation. The correct approach involves using the washer method, where the area A is defined as A=π(r₁² - r₂²), with r₁ and r₂ representing the distances from the x-axis. The volume element is expressed as dV=A*dx, emphasizing the need to accurately determine the area of the washers formed.

PREREQUISITES
  • Understanding of integral calculus, specifically volume calculations using integration.
  • Familiarity with the washer method for calculating volumes of revolution.
  • Knowledge of the curves y=x² and y=2x and their intersections.
  • Basic proficiency in manipulating definite integrals and applying the Fundamental Theorem of Calculus.
NEXT STEPS
  • Study the washer method in detail to understand its application in volume calculations.
  • Learn how to set up and evaluate definite integrals for volumes of solids of revolution.
  • Explore examples of calculating volumes using the disk method for comparison.
  • Review the Fundamental Theorem of Calculus to reinforce integration techniques.
USEFUL FOR

Students in calculus courses, educators teaching integration techniques, and anyone interested in understanding the application of integration in calculating volumes of solids of revolution.

zcabral
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Find the volume of the solid formed by rotating the region inside the first quadrant enclosed by
y=x^2
y=2x
about the x-axis.
So i used intergral 0 to 2 (2x-x^2)^2 dx
this is wat i got...
pi*(((2^5)/5)-(4(2^4)/4)+(4(2^3)/3))
its wrong so wat did i do wrong?:confused:
 
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That region will make a washer. Your interval [0,2] looks right.

However it might be beneficial to find a small piece of volume of the actual shape.

V=A*h so an infinitely small piece of volume would be dV = A*dh, or more approately in our case:

dV=A*dx

The area is pi*r^2 since it forms a washer, so [tex]A=\pi (r_1^2-r_2^2)[/tex]

Where r1 and r2 are the distances from the x-axis where r1>r2

The Area portion is what you did wrong, do you see where to go from there?

Next time you should post questions like this (non-conceptual) questions in the homework help.
 
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