Pls tell how orange part is comming..why not there n

At the second line of the solution you can see that ,there are actually (n+1) terms,like [(n+0)^(-2)+(n+1)^(-2)+(n+2)^(-2)+.....+(n+n)^(-2)].That is the reason behind the orange pa...

if any doubt then ask

see attached , it will clear your doubt