This seems too easy, but here's the question:
$x^2$ is $x + x + ...+ x$ (with $x$ terms). Its derivative is $1 + 1 + ... + 1$ (also $x$ terms). So the derivative of $x^2$ seems to be $x$.
And another expression: we know that if $y = nx$, then $y' = n$, so that if $y = x * x$ then $y' = x$.
But we know by formula that if $y = x^2$, then $y' = 2x$
So, how to prove $y' = x$ is wrong ?
Thanks