Question

(f+del f)(vector a+vector del a)-f.vectora
Lt del t -->0 -----------------------------
del t
in this the first terms make f.del vector a /del t
but last term is zero why.......??

Answer 1

Let me try to translate:
\[ \lim_{\delta t \rightarrow 0} \frac{(f+\delta f)(\vec{a} + \delta \vec{a}) - f\cdot \vec{a}}{\delta t} \]

Now, this doesn't actually make sense, so if you would be so kind, please clarify what this refers to and what in the above expression that I wrote is incorrectly translated.