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**Please please, I need solution**

hello every one

I have this summation in my book

infinity

[tex]\sum[/tex] k*z

^{-k}

k=0

my solution:

infinity

[tex]\sum[/tex] k*z

^{-k}= z

^{-1}+2z

^{-2}+3z

^{-3}+...

k=0

please i want the closed form (step-by-step)

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