If $m$ is real number and $dx$ is infinitesimal, then $m+dx=m$ (if $dx$ is infinitesimal and $dx^2$ is infinitesimal, then $dx+dx^2=dx$).
I suppose that $m+dx=m$, because if $\dfrac{m}{dx}=N$ ($N$-infinitely large quantity), then $m+dx=N\cdot dx+dx=m$. Is it true?
Thanks.