Integration by Substitution. The rate of growth of Grass carp, one of the most common type of fish, was modelled by the equation 4x + 8 f(t) = x- + 4x + 4 where t is measured in years and f(t) in kilograms per year. Consider the integral 3 4x + 8 - dx x2 + 4x + 4 After substitution into u, the new integral can be written (a) The value of A is 0 2 O x O 4x O 4x + 2 Ox + 2 0 4 Oo 0 8 (b) The upper limit of integration b is O 25 O 1 O -9 O -25 0 3 O 50 O 18 0 9