If xyyx = 1, then what is  at (1, 1) equal to?

This question was previously asked in
NDA 01/2022: Maths Previous Year paper (Held On 10 April 2022)
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  1. -1
  2. 0
  3. 1
  4. 4

Answer (Detailed Solution Below)

Option 1 : -1
Free
NDA 01/2025: English Subject Test
30 Qs. 120 Marks 30 Mins

Detailed Solution

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Formula used :

  1. log mn = log m + log n 
  2. log mn = n log m
  3. Differentiation by part: 

 

Calculation :

Given that

xyyx = 1

Taking the log on both sides

log( xyyx) = log 1

Since, log mn = log m + log n 

⇒ log xy +  log yx = log 1

Since log mn = n log m

⇒ y log x + x log y = 0        (∵ log 1 = 0)

Diff. with resepct to x

 (y log x) +  (x log y) = 0 

Using differentiation by part

⇒ 

By using the formula (4) & (5)

⇒ 

⇒ 

⇒ 

Put x = y = 1 in above differential

⇒ 

  at (1, 1) equal to -1.

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