This question already has an answer here:
We know $x.x=x^2$(Consider $x\ne 0$)
$x.x$ is adding $x$ $x$ times. So we have
$x+x+\dots+x$($x$ times $x$ is added)=$x^2$......(1)
Differentiating both sides of (1) we get,
Find out the mistake in this proof.