The charge flowing through a resistance R varies
with time t as Q = at - ``bt^2``, where a and b are positive
constants. The total heat produced in R is:
(1)``\frac {a^3R}{6b}``
(2)``\frac {a^3R}{3b}``
(3) ``\frac {a^3R}{2b}``
(4)``\frac {a^3R}{b}``