The last term be n(n + 1)

The generalized equation be

……………….(1)

Since 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

We know

Thus substituting in (1)