Logarithm Addition Practice Problems & Rules

Master adding logarithms with same and different bases. Practice problems with step-by-step solutions and change of base formula examples.

📚Master Logarithm Addition with Interactive Practice
  • Apply the rule log_a(x) + log_a(y) = log_a(xy) with confidence
  • Convert logarithms to different bases using the change of base formula
  • Solve complex addition problems with logarithms of different bases
  • Simplify logarithmic expressions by combining terms correctly
  • Practice real-world applications of logarithm addition properties
  • Build fluency with logarithmic rules through targeted exercises

Understanding The Sum of Logarithms

Complete explanation with examples

Addition of Logarithms

The definition of a logarithm is:


logax=blog_a⁡x=b
X=abX=a^b

Where:
aa is the base of the exponent
XX is what appears inside the log, can also appear in parentheses
bb is the exponent we raise the log base to in order to get the number that appears inside the log.

Adding logarithms with the same base is based on the following rule:


logax+logay=loga(xy)log_a⁡x+log_a⁡y=log_a⁡(x\cdot y)

Visual explanation of logarithmic rules showing log(x·y) equals log(x) plus log(y), and log(x/y) equals log(x) minus log(y), with arrows connecting each part for clarity.

Adding logarithms with different bases is done by changing the base of the log using the following rule:

logaX=logbase we want to change toXlogbase we want to change toalog_aX=\frac{log_{base~we~want~to~change~to}X}{log_{base~we~want~to~change~to}a}

Logarithmic change of base formula illustrated: log base b of a equals log base x of a divided by log base x of b, with arrows showing transformation from original form.

Detailed explanation

Practice The Sum of Logarithms

Test your knowledge with 6 quizzes

\( \log4x+\log2-\log9=\log_24 \)

?=x

Examples with solutions for The Sum of Logarithms

Step-by-step solutions included
Exercise #1

log103+log104= \log_{10}3+\log_{10}4=

Step-by-Step Solution

To solve this problem, we will use the property of logarithms that allows us to combine the sum of two logarithms:

  • Step 1: Identify the formula. We use the property logb(x)+logb(y)=logb(xy)\log_b(x) + \log_b(y) = \log_b(x \cdot y) where both logarithms must have the same base.
  • Step 2: Recognize the base. Here, both logarithms are in base 10: log103\log_{10}3 and log104\log_{10}4.
  • Step 3: Apply the property. Add the two logarithms using the formula: log103+log104=log10(34)\log_{10}3 + \log_{10}4 = \log_{10}(3 \cdot 4).
  • Step 4: Perform the multiplication. Compute 343 \cdot 4 to get 12.
  • Step 5: Express the result as a single logarithm: log1012\log_{10}12.

Therefore, the expression log103+log104\log_{10}3 + \log_{10}4 simplifies to log1012\log_{10}12.

Answer:

log1012 \log_{10}12

Video Solution
Exercise #2

log24+log25= \log_24+\log_25=

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Identify the given expression as log24+log25 \log_2 4 + \log_2 5 .
  • Step 2: Use the sum of logarithms rule to simplify the expression.
  • Step 3: Calculate the product and express the result.

Let's work through each step:

Step 1: We have log24+log25 \log_2 4 + \log_2 5 as our expression.

Step 2: Apply the sum of logarithms formula:

log24+log25=log2(45) \log_2 4 + \log_2 5 = \log_2 (4 \cdot 5)

Step 3: Calculate the product:

4×5=20 4 \times 5 = 20

Thus, log2(45)=log220 \log_2 (4 \cdot 5) = \log_2 20 .

Therefore, the solution to the problem is log220 \log_2 20 .

Answer:

log220 \log_220

Video Solution
Exercise #3

log974+log912= \log_974+\log_9\frac{1}{2}=

Step-by-Step Solution

To solve this problem, we'll apply the following steps:

  • Step 1: Identify given logarithms and their base.
  • Step 2: Employ the sum of logarithms property to combine the terms.
  • Step 3: Calculate the resulting argument of the logarithm.

Now, let's work through each step:

Step 1: We have two logarithms: log974\log_9 74 and log912\log_9 \frac{1}{2}, sharing the base of 99.

Step 2: Since the bases are the same, we use the sum property of logarithms:

log974+log912=log9(74×12)\log_9 74 + \log_9 \frac{1}{2} = \log_9 (74 \times \frac{1}{2}).

Step 3: Calculate the product 74×1274 \times \frac{1}{2}:

74×12=3774 \times \frac{1}{2} = 37.

So, we have:

log9(74×12)=log937\log_9 (74 \times \frac{1}{2}) = \log_9 37.

Therefore, the solution to the problem is log937\log_9 37.

Answer:

log937 \log_937

Video Solution
Exercise #4

2log82+log83= 2\log_82+\log_83=

Step-by-Step Solution

2log82=log822=log84 2\log_82=\log_82^2=\log_84

2log82+log83=log84+log83= 2\log_82+\log_83=\log_84+\log_83=

log843=log812 \log_84\cdot3=\log_812

Answer:

log812 \log_812

Video Solution
Exercise #5

3log49+8log413= 3\log_49+8\log_4\frac{1}{3}=

Step-by-Step Solution

Where:

3log49=log493=log4729 3\log_49=\log_49^3=\log_4729

y

8log413=log4(13)8= 8\log_4\frac{1}{3}=\log_4\left(\frac{1}{3}\right)^8=

log4138=log416561 \log_4\frac{1}{3^8}=\log_4\frac{1}{6561}

Therefore

3log49+8log413= 3\log_49+8\log_4\frac{1}{3}=

log4729+log416561 \log_4729+\log_4\frac{1}{6561}

logax+logay=logaxy \log_ax+\log_ay=\log_axy

(72916561)=log419 \left(729\cdot\frac{1}{6561}\right)=\log_4\frac{1}{9}

log491=log49 \log_49^{-1}=-\log_49

Answer:

log49 -\log_49

Video Solution

Frequently Asked Questions

How do you add logarithms with the same base?

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When adding logarithms with the same base, use the rule: log_a(x) + log_a(y) = log_a(xy). Simply multiply the numbers inside the logarithms while keeping the same base. For example, log_8(32) + log_8(2) = log_8(32 × 2) = log_8(64) = 2.

Can you add logarithms with different bases directly?

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No, you cannot add logarithms with different bases directly. You must first convert them to the same base using the change of base formula: log_a(x) = log_b(x)/log_b(a). Then apply the addition rule for same-base logarithms.

What is the change of base formula for logarithms?

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The change of base formula is: log_a(X) = log_c(X)/log_c(a), where 'c' is the new base you want to convert to. This allows you to rewrite any logarithm in terms of a different base, making calculations easier.

Why does log_a(x) + log_a(y) equal log_a(xy)?

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This property comes from the exponential relationship. If log_a(x) = m and log_a(y) = n, then a^m = x and a^n = y. Therefore, xy = a^m × a^n = a^(m+n), which means log_a(xy) = m + n = log_a(x) + log_a(y).

What are common mistakes when adding logarithms?

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Common mistakes include: 1) Adding logarithms with different bases without converting first, 2) Adding the numbers inside instead of multiplying them, 3) Forgetting that the addition rule only works for multiplication inside the log, not addition. Always verify bases match before applying rules.

How do you solve equations with logarithm addition?

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Steps to solve: 1) Convert all logarithms to the same base if needed, 2) Use the addition rule to combine terms: log_a(x) + log_a(y) = log_a(xy), 3) Simplify the expression inside the logarithm, 4) Convert back to exponential form to solve for the variable.

When should I use the change of base formula?

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Use the change of base formula when: 1) Adding logarithms with different bases, 2) Your calculator only has log base 10 or natural log functions, 3) Converting to a base that makes calculations easier. Choose the smallest or most convenient base for conversion.

What's the difference between log addition and log subtraction rules?

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Addition rule: log_a(x) + log_a(y) = log_a(xy) - you multiply inside the log. Subtraction rule: log_a(x) - log_a(y) = log_a(x/y) - you divide inside the log. Remember: addition becomes multiplication, subtraction becomes division.

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