Two cars P and Q start from a point at the same
time in a straight line and their positions are
represented by ``x_p(t) = at + bt^2`` and ``x_Q (t) = ft - t^2``.
At what time do the cars have the same velocity ?
(1)``\frac {a+ f}{2(1+ b)}``
(2)``\frac { f -a}{2(1+ b)}``
(3)``\frac {a- f}{(1+ b)}``
(4)``\frac {a+ f}{2(b - 1)}``