Cho 3a = 5. Khi đó log2581 bằng

C

Có 3^a = 5 Û a = log₃5.

log₂₅81\( = {\log _{{5^2}}}{3^4}\)\( = \frac{4}{2}{\log _5}3\)\( = \frac{2}{{{{\log }_3}5}} = \frac{2}{a}\).