Tính:

a) log₃ 5. log₅ 7 .log₇ 9;                          

b) ${\log }_2\frac{1}{25}.{\log }_3\frac{1}{32}.{\log }_5\frac{1}{27}$

a) ${\log }_{3}5.{\log }_{5}7.{\log }_{7}9$  

$={\log }_{3}5.\frac{{\log }_{3}7}{{\log }_{3}5}.\frac{{\log }_{3}9}{{\log }_{3}7}$

b) ${\log }_2\frac{1}{25}.{\log }_3\frac{1}{32}.{\log }_5\frac{1}{27}={\log }_2{5}^{-2}.{\log }_3{2}^{-5}.{\log }_5{3}^{-3}$

<=> (–2) log₂ 5. (–5)log₃ 2. (–3) . (–3) log₅ 3

 <=> –30 log₂ 5 . log₂ 3 . log₅ 3 log₇ 21

$\iff -30{\log }_{2}5.\frac{{\log }_{2}2}{{\log }_{2}3}.\frac{{\log }_{2}3}{{\log }_{2}5}=-30$