answer yaaro-3

1)you cannot differentiate.. because this function is defined only at some points...
it is not even a continuous function.
so there is no question of taking derivatives
2)Secondly because the number of terms also vary with x

1=1/x+1/x+1/x+1/x+....+1/x x times

differentiating both sides

0=-1/x2+(-1/x2)+(-1/x2)+....+(-1/x2) x times

0=-1/x
this could only be a possibillity but only possible if the above condition is not taken into consideration

You can differentiate wrt to a constant variable..not a constant.

http://targetiit.com/iit-jee-forum/posts/progression-10598.html