If log 10 5 + log 10 ( 5 𝑥 + 1 ) = log 10 ( 𝑥 + 5 ) + 1 log 10 5+log 10 (5x+1)=log 10 (x+5)+1, find 𝑥 x.
Aptitude
Logarithm
Difficulty: Medium
Choose an option
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A2
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B3
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C4
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D5
Answer
Correct Answer: 3
Explanation
Given data & domain
- 5x+1 > 0, x+5 > 0 (log arguments must be positive).
Concept / Approach
- Use log properties: log 𝑎 + log 𝑏 = log ( 𝑎 𝑏 ) loga+logb=log(ab) and 1 = log 10 10 1=log 10 10.
Step-by-step calculation log 10 [ 5 ( 5 𝑥 + 1 ) ] = log 10 [ 10 ( 𝑥 + 5 ) ] log 10 [5(5x+1)]=log 10 [10(x+5)] ⇒ 25 𝑥 + 5 = 10 𝑥 + 50 ⇒25x+5=10x+50 15 𝑥 = 45 ⇒ 𝑥 = < 𝑚 𝑎 𝑟 𝑘 > 3 < / 𝑚 𝑎 𝑟 𝑘 > 15x=45⇒x=3
Verification5x+1 = 16 > 0, x+5 = 8 > 0 valid. Substitute to check equality.
Final Answer3