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Question

Let K be the set of all real values of x where the function f(x)=sin|x||x|+2(xπ)cos|x| is not differentiable. Then the set K is equal to:

A
{π}
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B
{0}
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C
ϕ(anemptyset)
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D
{0,π}
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Solution

The correct option is C ϕ(anemptyset)

Differentiability -

Let f(x) be a real valued function defined on an open interval (a, b) and x\epsilon (a, b).Then the function f(x) is said to be differentiable at x_{\circ } if

\lim_{h\rightarrow 0}\:\frac{f(x_{0}+h)-f(x_{0})}{(x_{0}+h)-x_{0}}


or\:\:\:\lim_{x\rightarrow x_{0}}\:\frac{f(x)-f(x_{0})}{x-x_{0}}

-

Checking differentiability at x=0

for x>0,

f(x)=\sin x-x+2(x-\pi)\cos x

f{}'(x)=\cos x-1+2\cos x-2(x-\pi)\sin x

RHD = f{}'(0+)=1-1+2-2(-\pi)\cdot 0=2

for x<0,

f(x)=-\sin x+x+2(x-\pi)\cos x

f{}'(x)=-\cos x+1+2\cos x-2(x-\pi)\sin x

LHD = f{}'(0-)=-1+1+2-2(-\pi)\cdot 0=2

\because LHD = RHD

differentiable at x = 0 => differentiable everywhere


So, option 2 correct




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