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Question

Let $a = 30!$, $b = 50!$, and $c = 100!$. Consider the following numbers: 

$log_a c$,           $log_c a$,           $log_b a$,             $log_a b$ 

Which one of the following inequalities is CORRECT?

The correct answer is
$log_c a < log_b a < log_a b < log_a c$

To solve this problem, we need to evaluate the given logarithmic expressions and determine which of the provided inequalities is correct. Let's analyze each expression:

We are given:

  • \(a = 30!\)\(b = 50!\)\(c = 100!\)
  • Expressions to evaluate: \(\log_a c\)\(\log_c a\)\(\log_b a\)\(\log_a b\)

Let's compute each logarithm using the property \(\log_x y = \frac{\log y}{\log x}\):

  1. \(\log_a c = \frac{\log(100!)}{\log(30!)}\)
    Since \(100! > 30!\), this is a large number.
  2. \(\log_c a = \frac{\log(30!)}{\log(100!)}\)
    Since \(30! < 100!\), this is a small number.
  3. \(\log_b a = \frac{\log(30!)}{\log(50!)}\)
    Since \(30! < 50!\), this is a relatively small number.
  4. \(\log_a b = \frac{\log(50!)}{\log(30!)}\)
    Since \(50! > 30!\), this is a relatively large number.

Now, let's write out the approximate comparative sizes based on the factorial properties:

  • \(\log_c a\) will be the smallest because \(\log(30!) \ll \log(100!)\).
  • \(\log_b a\) will be larger than \(\log_c a\) but smaller than \(\log_a b\).
  • \(\log_a b\) will be next since \(\log(50!) > \log(30!)\).
  • \(\log_a c\) will be the largest because \(\log(100!) \gg \log(30!)\).

This order matches the first option provided:

\(\log_c a < \log_b a < \log_a b < \log_a c\)

Therefore, the correct inequality is:

\(\log_c a < \log_b a < \log_a b < \log_a c\)

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Important Questions from Logarithms

  1. Real numbers $y$, $p$, and $n$ (all greater than 1) satisfy
    $$(\log_{p^{1/n}} y)(\log_{y^{1/n}} p) = 16,$$
    where the logarithms are taken to the bases $p^{1/n}$ and $y^{1/n}$.
    The value of $n$ is ________
  2. Consider two distinct positive real numbers $m, n$, with $m > n$.

    Let $x = n^{\log_{10}(m)}$ and $y = m^{\log_{10}(n)}$. The relation between $x$ and $y$ is _______.

  3. If $\log_x (5/7) = -1/3$, then the value of $x$ is
  4. A value of x that satisfies the equation $ \log x + \log (x - 7) = \log (x + 11) + \log 2 $ is
  5. For a real number $x > 1$, 
    $\frac{1}{\log_2 x} + \frac{1}{\log_3 x} + \frac{1}{\log_4 x} = 1$ 
    The value of $x$ is

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