It almost looks like a cup

Calculus Level 3

The area enclosed by the curve y = ln ( x 3 ) y = \ln(x^{3}) , y = 1 y=1 and x x -axis is revolved about the y y -axis to form a solid.

The volume of the solid formed can be expressed as A π ( e B 1 ) A\pi(e^{B}-1) . Find A B AB .


The answer is 1.

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1 solution

Sam Sam
Mar 22, 2020

let's rotate the shape 90 degrees, so it's easyer to work with.

y = ln ( x 3 ) e y 3 = x y=\ln(x^{3} ) ⇒ e^{\frac{y}{3}} = x

using a b A ( x ) d x = π a b f ( x ) 2 d x \displaystyle\int_{a}^{b} A(x) dx = \pi\displaystyle\int_{a}^{b} f(x)^{2} dx

π 0 1 e 2 y 3 d y 3 π 2 0 1 2 3 e 2 y 3 d y \pi\displaystyle\int_{0}^{1} e^{\frac{2y}{3}} dy ⇒ \frac{3\pi}{2}\displaystyle\int_{0}^{1} \frac{2}{3}e^{\frac{2y}{3}} dy

3 π 2 e 2 y 3 0 1 π 3 2 ( e 2 3 1 ) \frac{3\pi}{2}e^{\frac{2y}{3}}|^{1}_{0} ⇒ \pi\frac{3}{2}(e^{\frac{2}{3}}-1)

A B = 3 2 2 3 = 1 AB = \frac{3}{2}\frac{2}{3} = \boxed{1}

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