Ta có
$\dfrac{a}{b} = \dfrac{b}{c} = \dfrac{c}{d} = \dfrac{d}{e}$ nên ta có
$\begin{cases}
\dfrac{a}{b} = \dfrac{b}{c}\\
\dfrac{a}{b} = \dfrac{c}{d}\\
\dfrac{a}{b} = \dfrac{d}{e}
\end{cases}$
Vậy
$VT = \left( \dfrac{a}{b} \right)^4 = \dfrac{a}{b}.\dfrac{a}{b}.\dfrac{a}{b}.\dfrac{a}{b}$
$= \dfrac{a}{b}. \dfrac{b}{c} . \dfrac{c}{d} . \dfrac{d}{e}$
$ = \dfrac{a}{e} = VP$
