Đặt \(t = {5^x}\), ta có:
\(\eqalign{
& {1 \over 5}{t^2} + 5t = 250 \Leftrightarrow {t^2} + 25t - 1250 = 0 \cr
& \Leftrightarrow \left[ \matrix{
t = 25 \hfill \cr
t = - 50\,\,(loai) \hfill \cr} \right. \cr
& \Leftrightarrow {5^x} = 25 \Leftrightarrow x = 2 \cr}\)
