see,

x+x+x+x.......................................................xtimes=x^2

differentiating wrt x

1+1+1...................1 times =2 x

x=2x    how is this possible?

x can be written as x^2 only if x is a natural no.

sfor example you can't add 2/3 2/3 times

hence on differentiating you will get 0=0