Limit of series

Calculus Level 5

f ( x ) = r = 1 n tan ( x 2 r ) sec ( x 2 r 1 ) ; r , n N g ( x ) = { lim n ln ( f ( x ) + tan ( x 2 n ) ) ( f ( x ) + tan ( x 2 n ) ) n sin ( tan ( x 2 ) ) 1 + ( f ( x ) + tan ( x 2 n ) ) n x π 4 K x = π 4 \displaystyle f(x)=\sum _{r=1}^n \tan \left(\frac x{ 2^r} \right) \sec \left( \frac x{2^{r-1}}\right); \quad r,n \in \mathbb{N} \\ g(x)=\begin{cases} \displaystyle \lim _{ n \to \infty} \frac {\ln(f(x)+\tan(\frac { x }{ { 2 }^{ n } } ))-(f(x)+\tan(\frac { x }{ { 2 }^{ n } } ))^{ n }\lfloor \sin(\tan(\frac { x }{ 2 } )) \rfloor}{ 1+{ (f(x)+\tan(\frac { x }{ { 2 }^{ n } } )) }^{ n } } & x\ne \frac \pi 4 \\ K & x= \frac \pi 4 \end{cases}

Given the above and that the domain of g ( x ) g(x) is ( 0 , π 2 ) \left(0, \frac \pi 2 \right) .

  • Find the value of K K such that g ( x ) g(x) is continuous at x = π 4 x=\frac \pi 4 .
  • Also find M M the number of points of discontinuity of g ( x ) g(x) in ( 0 , π 4 ) \left(0, \frac \pi 4 \right) .
  • Enter the answer as value of K 2 + M 2 K^2+M^2 . If you think K K does not exist enter the answer as 111 111 .

Notation: \lfloor \cdot \rfloor denotes the floor function .


The answer is 0.

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1 solution

Sumanth R Hegde
Dec 20, 2016

Consider f ( x ) f(x) . On adding t a n ( x / 2 n ) tan(x/2^n) with the last term of the summation, we get t a n ( x / 2 n 1 ) tan(x/2^{n-1}) ....add this with the next term of the summation and so on........you will notice that the whole summation gets reduced and we will end up with f ( x ) f(x) = t a n x t a n ( x / 2 n ) tanx - tan(x/2^n) After evaluating f ( x ) f(x) , g ( x ) g(x) be evaluated easily . From the condition of continuity of g ( x ) g(x) at x = ( π / 4 ) (\pi/4) , we get K = 0 \color{#3D99F6}\boxed {K=0} We will finally end up with K = 0 \color{#20A900}\boxed{K =0} and M = 0 \color{#20A900}\boxed{M=0} . Thus, K 2 + M 2 = 0 \color{#D61F06}\boxed {K^2 + M^2 = 0 }

well, yeah!

very colourful as well :P

Rohith M.Athreya - 4 years, 5 months ago

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