Two points $A$ and $B$ move from rest along a straight line with constant acceleration $f$ and $f'$ respectively. If $A$ takes $m$ sec. more than $B$ and describes '$n$' units more than $B$ in acquiring the same speed then\\
(1) $\left(f - f'\right)m^2 = ff'n$\\
(2) $\left(f + f'\right)m^2 = ff'n$\\
(3) $\frac{1}{2}\left(f + f'\right)m = ff'n^2$\\
(4) $\left(f' - f\right)n = \frac{1}{2}ff'm^2$