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Understanding and evaluating logarithms
Learn step-by-step with practice exercises built right in.
A logarithm is the inverse of an exponential function.
Evaluate: log₂ 8
Step 1: Understand the question: log₂ 8 means "2 to what power equals 8?"
Step 2: Find the power: 2¹ = 2 2² = 4 2³ = 8
Step 3: Answer: Since 2³ = 8, we have log₂ 8 = 3
Answer: 3
Evaluate:
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Read as: "log base of equals "
Example: because
Common log: means
Natural log: means where
Product Rule:
Quotient Rule:
Power Rule:
Change of Base:
We need to find:
This asks: "3 to what power equals 81?"
Since :
Answer:
Evaluate: log₁₀ 1000
Step 1: Rewrite as an exponential equation: log₁₀ 1000 = x means 10ˣ = 1000
Step 2: Express 1000 as a power of 10: 1000 = 10³
Step 3: Therefore: log₁₀ 1000 = 3
Step 4: Note: log₁₀ is called the "common logarithm" Often written as just "log" without the base
Answer: 3
Expand using log properties:
Use quotient, product, and power rules:
Step 1: Apply quotient rule
Convert to logarithmic form: 5³ = 125
Step 1: Recall the relationship: bˣ = y is equivalent to logᵦ y = x
Step 2: Identify the parts: Base (b) = 5 Exponent (x) = 3 Result (y) = 125
Step 3: Write in logarithmic form: log₅ 125 = 3
Step 4: Verify: "5 to what power equals 125?" 5³ = 125 ✓
Answer: log₅ 125 = 3
Simplify using logarithm properties: log₃ 27 + log₃ 9
Step 1: Use the product rule: logᵦ m + logᵦ n = logᵦ(mn)
Step 2: Apply the rule: log₃ 27 + log₃ 9 = log₃(27 · 9) = log₃ 243
Step 3: Evaluate log₃ 243: What power of 3 equals 243? 3¹ = 3 3² = 9 3³ = 27 3⁴ = 81 3⁵ = 243
Step 4: Therefore: log₃ 243 = 5
Alternative - evaluate first: log₃ 27 = 3 (since 3³ = 27) log₃ 9 = 2 (since 3² = 9) 3 + 2 = 5 ✓
Answer: 5
Solve:
Step 1: Use product rule (combine logs)
Expand using logarithm properties: log₂(8x³/y²)
Step 1: Apply the quotient rule: logᵦ(m/n) = logᵦ m - logᵦ n
log₂(8x³/y²) = log₂(8x³) - log₂(y²)
Step 2: Apply the product rule to first term: logᵦ(mn) = logᵦ m + logᵦ n
log₂(8x³) = log₂ 8 + log₂ x³
Step 3: Apply the power rule: logᵦ(mⁿ) = n logᵦ m
log₂ x³ = 3 log₂ x log₂ y² = 2 log₂ y
Step 4: Combine all parts: log₂(8x³/y²) = log₂ 8 + 3 log₂ x - 2 log₂ y
Step 5: Simplify log₂ 8: log₂ 8 = 3 (since 2³ = 8)
Step 6: Final answer: 3 + 3 log₂ x - 2 log₂ y
Answer: 3 + 3 log₂ x - 2 log₂ y
Step 2: Apply product rule to first term
Step 3: Apply power rule
Answer:
Step 2: Convert to exponential form
Step 3: Simplify left side (difference of squares)
Step 4: Solve for x
Step 5: Check both solutions
Answer: