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Find dy/dx for the given function: (cos x)^y = (cos y)^x

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Question

Find `bb(dy/dx)` for the given function:

(cos x)y = (cos y)x

Sum
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Solution

Given, (cos x)y = (cos y)x

Taking logarithm of both sides,

log (cos x)y = log (cos y)x

y log cos x = x log cos y

Differentiating both sides with respect to x,

`y d/dx log cos x + log cos x d/dx (y)= x d/dx log cos y + log cos y d/dx (x)`

⇒ `y * 1/(cos x) d/dx cos x + log cos x * dy/dx= x * 1/(cos y) d/dx cos y + log cos y xx 1`

⇒ `y * 1/(cos x) (- sin x) + log cos x. dy/dx = x 1/(cos y) (-sin y) dy/dx + log cos y`

⇒ `-y tan x + log cos x dy/dx = - x tan y dy/dx + log cos y`

⇒ `log cos x dy/dx + x tan y dy/dx = log cos y + y tan x`

⇒ `dy/dx (log cos x + x tan y) = log cos y + y tan x`

`therefore dy/dx = (log cos y + y tan x)/ (log cos x + x tan y)`

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Chapter 5: Continuity and Differentiability - Exercise 5.5 [Page 178]

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NCERT Mathematics Part 1 and 2 [English] Class 12
Chapter 5 Continuity and Differentiability
Exercise 5.5 | Q 14 | Page 178

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