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Question

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

The correct answer is
24

Logarithm Equation Solving

The problem requires finding the value of $x$ for the equation:

$ \frac{1}{\log_2 x} + \frac{1}{\log_3 x} + \frac{1}{\log_4 x} = 1 $

We are given the condition $x > 1$. The solution involves applying key logarithm properties.

Logarithm Properties Applied

  • Change of Base Formula: $ \frac{1}{\log_a b} = \log_b a $. This allows us to change the base of the logarithm to $x$.
  • Sum Rule for Logarithms: $ \log_b m + \log_b n = \log_b (mn) $. This helps combine multiple logarithm terms with the same base.

Solution Steps

  1. Transform the equation: Using the change of base formula, rewrite each term with base $x$: $ \log_x 2 + \log_x 3 + \log_x 4 = 1 $
  2. Combine logarithmic terms: Apply the sum rule for logarithms: $ \log_x (2 \times 3 \times 4) = 1 $ Simplify the product inside the logarithm: $ \log_x 24 = 1 $
  3. Convert to exponential form: Recall that $ \log_b a = c $ is equivalent to $ b^c = a $. Applying this definition: $ x^1 = 24 $
  4. Determine the value of x: From the exponential form, we find: $ x = 24 $ The obtained value $x=24$ satisfies the initial condition $x > 1$.
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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. 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?

  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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