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Logarithmic Functions

Understanding and evaluating logarithms

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Logarithmic Functions

Definition

A logarithm is the inverse of an exponential function.

log⁡b(x)=ymeansby=x\log_b(x) = y \quad \text{means} \quad b^y = x

Read as: "log base bb of xx equals yy"

Example: log⁡2(8)=3\log_2(8) = 3 because 23=82^3 = 8

Common Logarithms

Common log: log⁡(x)\log(x) means log⁡10(x)\log_{10}(x)

Natural log: ln⁡(x)\ln(x) means log⁡e(x)\log_e(x) where e≈2.718e \approx 2.718

Properties of Logarithms

Product Rule: log⁡b(MN)=log⁡b(M)+log⁡b(N)\log_b(MN) = \log_b(M) + \log_b(N)

Quotient Rule: log⁡b(MN)=log⁡b(M)−log⁡b(N)\log_b\left(\frac{M}{N}\right) = \log_b(M) - \log_b(N)

Power Rule: log⁡b(Mp)=p⋅log⁡b(M)\log_b(M^p) = p \cdot \log_b(M)

Change of Base: log⁡b(x)=log⁡(x)log⁡(b)\log_b(x) = \frac{\log(x)}{\log(b)}

Special Values

  • log⁡b(1)=0\log_b(1) = 0 (because b0=1b^0 = 1)
  • log⁡b(b)=1\log_b(b) = 1 (because b1=bb^1 = b)
  • log⁡b(bx)=x\log_b(b^x) = x
  • blog⁡b(x)=xb^{\log_b(x)} = x

📚 Practice Problems

1Problem 1easy

❓ Question:

Evaluate: log₂ 8

💡 Show Solution

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

2Problem 2easy

❓ Question:

Evaluate: log⁡3(81)\log_3(81)

💡 Show Solution

We need to find: log⁡3(81)=?\log_3(81) = ?

This asks: "3 to what power equals 81?"

3?=813^? = 81

Since 34=813^4 = 81: log⁡3(81)=4\log_3(81) = 4

Answer: 44

3Problem 3easy

❓ Question:

Evaluate: log₁₀ 1000

💡 Show Solution

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

4Problem 4medium

❓ Question:

Expand using log properties: log⁡5(x3yz2)\log_5(\frac{x^3y}{z^2})

💡 Show Solution

Use quotient, product, and power rules:

Step 1: Apply quotient rule log⁡5(x3yz2)=log⁡5(x3y)−log⁡5(z2)\log_5\left(\frac{x^3y}{z^2}\right) = \log_5(x^3y) - \log_5(z^2)

Step 2: Apply product rule to first term =log⁡5(x3)+log⁡5(y)−log⁡5(z2)= \log_5(x^3) + \log_5(y) - \log_5(z^2)

Step 3: Apply power rule =3log⁡5(x)+log⁡5(y)−2log⁡5(z)= 3\log_5(x) + \log_5(y) - 2\log_5(z)

Answer: 3log⁡5(x)+log⁡5(y)−2log⁡5(z)3\log_5(x) + \log_5(y) - 2\log_5(z)

5Problem 5medium

❓ Question:

Convert to logarithmic form: 5³ = 125

💡 Show Solution

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

6Problem 6medium

❓ Question:

Simplify using logarithm properties: log₃ 27 + log₃ 9

💡 Show Solution

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

7Problem 7hard

❓ Question:

Solve: log⁡2(x+3)+log⁡2(x−3)=4\log_2(x + 3) + \log_2(x - 3) = 4

💡 Show Solution

Step 1: Use product rule (combine logs) log⁡2[(x+3)(x−3)]=4\log_2[(x + 3)(x - 3)] = 4

Step 2: Convert to exponential form (x+3)(x−3)=24=16(x + 3)(x - 3) = 2^4 = 16

Step 3: Simplify left side (difference of squares) x2−9=16x^2 - 9 = 16

Step 4: Solve for x x2=25x^2 = 25 x=±5x = \pm 5

Step 5: Check both solutions

  • x=5x = 5: log⁡2(8)+log⁡2(2)=3+1=4\log_2(8) + \log_2(2) = 3 + 1 = 4 ✓
  • x=−5x = -5: log⁡2(−2)+log⁡2(−8)\log_2(-2) + \log_2(-8) ✗ (negative logs undefined)

Answer: x=5x = 5

8Problem 8hard

❓ Question:

Expand using logarithm properties: log₂(8x³/y²)

💡 Show Solution

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

Explain using:

⚠️ Common Mistakes: Logarithmic Functions

Avoid these 3 frequent errors

🌍 Real-World Applications: Logarithmic Functions

See how this math is used in the real world

📝 Worked Example: Solving a Quadratic by Factoring

Problem:

Solve x2−5x+6=0x^2 - 5x + 6 = 0.

2Factor the quadratic
3Set each factor equal to zero

📌 Related Topics in Exponential and Logarithmic Functions

❓ Frequently Asked Questions

What is Logarithmic Functions?▾
Understanding and evaluating logarithms
How can I study Logarithmic Functions effectively?▾
Start by reading the study notes and working through the examples on this page. Then use the flashcards to test your recall. Practice with the 8 problems provided, checking solutions as you go. Regular review and active practice are key to retention.
Is this Logarithmic Functions study guide free?▾
Yes — all study notes, flashcards, and practice problems for Logarithmic Functions on Study Mondo are free to access. No account is needed.
What course covers Logarithmic Functions?▾
Logarithmic Functions is part of the Algebra 2 course on Study Mondo, specifically in the Exponential and Logarithmic Functions section. You can explore the full course for more related topics and practice resources.
Are there practice problems for Logarithmic Functions?▾
Yes, this page includes 8 practice problems with detailed solutions. Each problem includes a step-by-step explanation to help you understand the approach.