Logarithms' challenger #1

Algebra Level 2

log 3 4 × log 4 5 × log 5 6 × log 6 7 × log 7 8 × log 8 9 = ? \log_{3}{4} \times \log_{4}{5} \times \log_{5}{6} \times \log_{6}{7} \times \log_{7}{8} \times \log_{8}{9}= \ ?


This problem is a part of this set .


The answer is 2.

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2 solutions

Jack Rawlin
Oct 13, 2015
  1. First we convert all of the logs into the same base using log a b = log c b log c a \log_{a}{b} = \frac{\log_{c}{b}}{\log_{c}{a}}

We'll convert to a base of 3 3 since that's the first base used in the question.

log 3 4 log 3 5 log 3 4 log 3 6 log 3 5 log 3 7 log 3 6 log 3 8 log 3 7 log 3 9 log 3 8 = ? \log_{3}{4} \cdot \frac{\log_{3}{5}}{\log_{3}{4}} \cdot \frac{\log_{3}{6}}{\log_{3}{5}} \cdot \frac{\log_{3}{7}}{\log_{3}{6}} \cdot \frac{\log_{3}{8}}{\log_{3}{7}} \cdot \frac{\log_{3}{9}}{\log_{3}{8}} = ?

  1. Simplifying this down gives us:

log 3 9 = ? \log_{3}{9} = ?

  1. We can then find the answer by simplifying further.

log 3 3 2 = ? \log_{3}{3^{2}} = ?

2 log 3 3 = ? 2\log_{3}{3} = ?

2 1 = ? 2 \cdot 1 = ?

? = 2 ? = 2

The answer is 2 \boxed{2}

Great explanation ! :D

Rohit Udaiwal - 5 years, 8 months ago
Kay Xspre
Oct 11, 2015

Using the base changing rule (here I will use natural logarithm as the new base), we will get that l o g a b = l n ( b ) l n ( a ) log_{a}b = \frac{ln(b)}{ln(a)} With this it is possible to see that the remaining terms will be l n ( 9 ) l n ( 3 ) = 2 l n ( 3 ) l n ( 3 ) = 2 \frac{ln(9)}{ln(3)}=\frac{2ln(3)}{ln(3)} = 2

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