# Question: andnbspandnbspheloandnbsp i get stuck in question 418andnbspi dont understand what...

###### Question details

Helo i get stuck in question 4.18

i dont understand what (2) this graph is drawing about,how does this related to the equation with double summation

in (1)(the black circle i draw)how does the author comes into conclusion such that r=1 to n --> r=1 to infinity

thank you