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Previous Year Question (PYQs)



The curve $y=\frac{x}{1+x\tan x}$ attains maxima





Solution

Given the curve 
$y = \dfrac{x}{1 + x \tan x}$ 
Differentiate using quotient rule: 
$y' = \dfrac{(1 + x \tan x) - x(\tan x + x \sec^2 x)}{(1 + x \tan x)^2}$ 
Simplify the numerator: 
$N = 1 + x \tan x - x \tan x - x^2 \sec^2 x$ 
$N = 1 - x^2 \sec^2 x$ 
Set $N = 0$ for maxima/minima: 
$1 - x^2 \sec^2 x = 0$ 
$x^2 \sec^2 x = 1$ 
$x \sec x = \pm 1$ 
Hence the curve attains maxima when: 
$x \sec x = 1$
$x=\cos x$


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