Solve Product of Logarithms: log₄(9) × log₁₃(7) Evaluation

Logarithm Products with Change of Base

log49×log137= \log_49\times\log_{13}7=

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:03 We'll use the formula for multiplication of logarithms
00:07 We'll exchange between the bases of the logarithms
00:13 We'll use this formula in our exercise
00:33 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

log49×log137= \log_49\times\log_{13}7=

2

Step-by-step solution

To solve the problem log49×log137 \log_49 \times \log_{13}7 , we'll employ the change of base formula for logarithms:

  • Step 1: Apply the change of base formula to each logarithm.
  • Step 2: Use logarithm properties and analyze transformations for a match with choices.

Now, let's work through each step:
Step 1: Use the change of base formula on each log:
log49=loga9loga4 \log_49 = \frac{\log_a 9}{\log_a 4} and log137=logb7logb13 \log_{13}7 = \frac{\log_b 7}{\log_b 13} , where a a and b b are arbitrary positive bases.
Both expressions use a common base not relevant for the solution but illustrate the transformation ability.

Step 2: We'll recombine and look for products that can utilize these, such as:

log139×log47 \log_{13}9\times\log_47 becomes loga9loga13×logb7logb4 \frac{\log_a 9}{\log_a 13} \times \frac{\log_b 7}{\log_b 4}
Applying cross multiplication or iteration forms, the structure aligns with the multiplication identity for this problem due to independence of base.

Therefore, the transformed expression satisfying the criteria is log139×log47 \log_{13}9\times\log_47 .

3

Final Answer

log139×log47 \log_{13}9\times\log_47

Key Points to Remember

Essential concepts to master this topic
  • Change of Base Formula: Transform logarithms to find equivalent expressions
  • Cross-Base Property: logab×logcd=logcb×logad \log_a b \times \log_c d = \log_c b \times \log_a d
  • Verify: Check that bases and arguments swap correctly in final answer ✓

Common Mistakes

Avoid these frequent errors
  • Adding bases and arguments instead of applying logarithm properties
    Don't write log49×log137=log17(16) \log_4 9 \times \log_{13} 7 = \log_{17}(16) by adding! Logarithms don't work like regular multiplication - you can't combine bases or arguments by addition. Always use the change of base formula to transform the expression into equivalent forms.

Practice Quiz

Test your knowledge with interactive questions

\( \log_{10}3+\log_{10}4= \)

FAQ

Everything you need to know about this question

Why can't I just multiply the bases and arguments separately?

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Logarithms follow special rules that are different from regular arithmetic. When you multiply logarithms with different bases, you need to use the change of base formula to find equivalent expressions, not simple multiplication!

How do I know which answer choice is correct?

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Look for the choice that swaps the bases and arguments correctly. In this case, log49×log137=log139×log47 \log_4 9 \times \log_{13} 7 = \log_{13} 9 \times \log_4 7 because the cross-multiplication property applies.

What is the change of base formula exactly?

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The change of base formula is: logab=logcblogca \log_a b = \frac{\log_c b}{\log_c a} for any valid base c. This lets you convert any logarithm to a different base system.

Can I use a calculator to check my answer?

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Yes! Calculate both log49×log137 \log_4 9 \times \log_{13} 7 and log139×log47 \log_{13} 9 \times \log_4 7 using the change of base formula with base 10. They should give the same decimal result.

Why does this cross-multiplication property work?

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When you apply the change of base formula to both logarithms and multiply them together, the intermediate base terms cancel out, leaving you with the swapped form. It's like algebraic cancellation!

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