Re: [問題]二技考古

[Eliphalet (落閘放狗)]

※ 引述《jkl (新手上路.....)》之銘言：
: sigma      n
:            ______
: 2->無限大  3^n
: 不太會解請幫忙






            ∞    x
考慮 f(x) = Σ ( --- )^n  ,  |x| < 3  ( radius of convergence 3 )
            n=1   3

              ∞    n
 =>  f'(x) =  Σ  ------ * x^(n-1)
              n=1   3^n



         x
       ------
         3                            3
 ( ---------------- )' |         =  -----
            x           x = 1         4
     1 -  -----
            3


      ∞    n        3
 =>   Σ  ------ =  ---
      n=1   3^n      4

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