The series is like...3n^{2} + 8rn + 4r^{2}
3n^{2} + 8rn + 4r^{2} = 3n^{2} + 6rn + 2rn + 4r^{2} = (3n + 2r)(n + 2r)
so take r/n = x and 1/n = dx...so the summation reduces to
\int_{0}^{1}}{\frac{dx}{(3 + 2x)(1 + 2x)}
which can be easily solved by using partial fractions...