Algebra - Logarithm

Logarithm

Understanding Logarithms: Definition, Properties, and Core Formulas

A logarithm is the inverse operation of exponentiation. The logarithm of a number \(N\) to the base \(b\), written as \(\log_b N\), represents the exponent to which the base must be raised to produce the number \(N\). That is:

\[ \log_b N = x \quad \text{means} \quad b^x = N \]

This is valid for: \( N > 0, b > 0, b \neq 1 \)

Key Properties of Logarithms

  1. Product Rule: \[ \log_b (N_1 N_2) = \log_b |N_1| + \log_b |N_2| \quad (N_1 N_2 > 0) \]
  2. Quotient Rule: \[ \log_b \left( \frac{N_1}{N_2} \right) = \log_b |N_1| - \log_b |N_2| \quad (N_1 N_2 > 0) \]
  3. Power Rule: \[ \log_b (N^\alpha) = \alpha \log_b N \quad (N > 0) \]
  4. Root Rule: \[ \log_b \sqrt[\alpha]{N} = \frac{1}{\alpha} \log_b N \quad (N > 0) \]
  5. Base Change Rule (Multiplicative): \[ \log_b N = \log_b a \cdot \log_a N \quad (a > 0, a \neq 1, N > 0) \]
  6. Base Change Rule (Fractional): \[ \log_b N = \frac{\log_a N}{\log_a b} \quad (a > 0, a \neq 1) \]
  7. Reciprocal Rule: \[ \log_b a = \frac{1}{\log_a b} \]
  8. Log of One: \[ \log_b 1 = 0 \]
  9. Log of Base: \[ \log_a a = 1 \]
  10. Log of Zero: \[ \log_b 0 \rightarrow \begin{cases} -\infty, & b > 1 \\ +\infty, & b < 1 \end{cases} \]
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