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A solid is generated by revolving about the x-axis the region bounded by the graph of the positive continuous function y = f(x), the x-axis, the fixed line x = a, and the variable line x = b, b > a. Its volume, for all b, is b2ab.b^{2}-a b. Find f(x).

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We have a solid generated by revolving about the xx-axis the region bounded by the graph of the positive continuous function y=f(x)y=f\left(x\right), the xx-axis, the fixed line x=ax=a, and the variable line x=b,b>ax=b,\enskip b>a. Its volume, for all bb, is b2ab.b^2-ab. We have to find f(x)f\left(x\right).

To find the volume of a solid, we need only observe that the cross-sectional area A(x)A(x) is the area of a disk of radius f(x)f(x), the distance of the planar region's boundary from the axis of revolution. Therefore, the volume of a solid is

V=πab[f(x)]2dx.\begin{align*} V=\pi\int_a^b\left[f\left(x\right)\right]^2dx. \end{align*}

We know that volume is b2abb^2-ab. So, we have

πax[f(t)]2dt=x2ax(for allx>a).\begin{align*} \pi\int_a^x\left[f\left(t\right)\right]^2\enskip dt=x^2-ax\quad\left(\text{for all}\enskip x>a\right). \end{align*}

Now, we can conclude that f(x)f\left(x\right) is

π[f(x)]2=ddx(x2ax)=2xaf(x)=2xaπ.\begin{align*} \pi\left[f\left(x\right)\right]^2=\dfrac{d}{dx}(x^2-ax)=2x-a\quad\Rightarrow\quad f\left(x\right)=\sqrt{\dfrac{2x-a}{\pi}}. \end{align*}

Hence, we get

f(x)=2xaπ.\boxed{f(x)=\sqrt{\dfrac{2x-a}{\pi}}.}

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