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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. If $\log_x (5/7) = -1/3$, then the value of $x$ is
  2. A value of x that satisfies the equation $ \log x + \log (x - 7) = \log (x + 11) + \log 2 $ is
  3. 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

  4. A petrified wood fossil was discovered with 8 g of $^{14}C$. The decay of $^{14}C$ over time is given by: 

    $N_T = N_0 e^{-0.0001216T}$ 

    If the half-life of $^{14}C$ is 5700 years, and the fossil initially had 32 g of $^{14}C$, the age of the fossil in years is ______.

  5. For integers $a$, $b$ and $c$, what would be the minimum and maximum values respectively of $a + b + c$ if $\log |a| + \log |b| + \log |c| = 0$?
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