Friday, 1 March 2019

sequences and series - Find sum of 1+costheta+frac12!cos2theta+cdots



Find the sum of following series:



1+cosθ+12!cos2θ+



where θR.



My attempt: I need hint to start.



Answer



Hint:
1+cosx+12!cos2x+=(e0ix+e1ix+12!e2ix+)=eeix



eix=cosx+isinxeeix=ecosxeisinx=ecosx(cos(sinx)+isin(sinx))
Your sum is

ecosxcos(sinx)


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