Lời giải 

* \( x^2 = \frac{27}{x}  \Rightarrow x = 3 \)  

* \( \frac{x^2}{27} = \frac{27}{x}  \Rightarrow x = 9 \)  

a)  \( V = \pi \int_0^3 \left( x^4 - \frac{x^4}{27^2} \right) dx + \pi \int_3^9 \left[ \left(\frac{27}{x}\right)^2 - \left(\frac{x^2}{27}\right)^2 \right] dx\)

b)  \(V = \pi \int_0^3 \left( \left(\sqrt{27y}\right)^2 - (\sqrt{y})^2 \right) dy + \pi \int_3^9 \left[ \left(\frac{27}{y}\right)^2 - (\sqrt{y})^2 \right] dy\)