The nth term be n(n + 1)(3n + 1)

1.2.4 + 2.3.7 + 3.4.10 + …………=

We know by property that:

∑axⁿ + bx^{n - 1} + cx^{n - 2}…….d₀=a∑xⁿ + b∑x^{n - 1} + c∑x^{n - 2}…….. + d₀∑1

…..(1)

We know

Thus from (1)