Question

Find the sum to n-terms of the series 1.2 + 2.3 + 3.4 + ............

Answer 1

Let T_r be the general term of the series 

So T_r = r(r + 1). 

To express t_r = f(r) – f(r–1) multiply and divide t_r by [(r + 2) – (r – 1)] 

Hence sum of series is f(n) – f(0).