If S1, S2, …., Sn are the sums of n terms of n G.P.’s whose first term is 1 in each and common ratios are 1, 2, 3, …., n respectively, then prove that
S1 + S2 + 2S3 + 3S4 + … (n – 1) Sn = 1n + 2n + 3n + … + nn.
S₁ = n [First term is 1, common ratio 1; so sum to n terms = 1 + 1 + 1 + - - = n]
ii) S₂ = (2ⁿ - 1)/(2 - 1) = (2ⁿ - 1)
iii) S₃ = (3ⁿ - 1)/2
iv) S₄ = (4ⁿ - 1)/3 ..
v) So, S₁ + S₂ + 2S₃ + 3S₄ + - - - - + (n - 1)Sⁿ =
= n + (2ⁿ - 1) + (3ⁿ - 1) + (4ⁿ - 1) + - - - - - - - + (nⁿ - 1)
= n + ( - 1 - 1 - 1 .... to n - 1 terms) + (2ⁿ + 3ⁿ + 4ⁿ + .... + nⁿ) =
= n - (n - 1) + (2ⁿ + 3ⁿ + 4ⁿ + .... + nⁿ) = 1 + (2ⁿ + 3ⁿ + 4ⁿ + .... + nⁿ)
= 1ⁿ + 2ⁿ + 3ⁿ + 4ⁿ + .... + nⁿ [Proved]