If 5x-1 = (2.5)log105, then what is the value of x ?

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CDS 02/2021: Maths Previous Paper (Held On 14 Nov 2021)
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  1. 1
  2. log102
  3. log10​5
  4. 2log10​5

Answer (Detailed Solution Below)

Option 4 : 2log10​5
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Detailed Solution

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Given:

5x-1 = (2.5)log105

Formula Used:

If ax = n then x = logan

logab = logeb/logea

Calculation:

We have 5x-1 = (2.5)log105

⇒ (2.5)log10= 5x-1 

⇒ log105  = log2.55x-1 

⇒ log105  = (x - 1) log2.55

⇒ (x - 1) = (log105)/(log2.55)

⇒ (x - 1) = log102.5

⇒ x = log102.5 + 1

⇒ x = log102.5 log1010

⇒ x = log1010 × 2.5

⇒ x = log1025

⇒ x = log1052

⇒ x = 2log10​5

∴ The value of x is 2log10​5.

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