# The sum of n terms of three arithmetical progression are S1 , S2 and S3 . The first term of each is unity and the common differences are 1, 2 and 3 respectively. Prove that S1 + S3 = 2S2 - Mathematics

Sum

The sum of n terms of three arithmetical progression are S1 , S2 and S3 . The first term of each is unity and the common differences are 1, 2 and 3 respectively. Prove that S1 + S3 = 2S2

#### Solution

We have,

S1 = Sum of n terms of an A.P. with first term 1 and common difference 1

= \frac { n }{ 2 } [2 × 1 + (n – 1) 1] = \frac { n }{ 2 } [n + 1]

S2 = Sum of n terms of an A.P. with first term 1 and common difference 2

= \frac { n }{ 2 } [2 × 1 + (n – 1) × 2] = n^2

S3 = Sum of n terms of an A.P. with first term 1 and common difference 3

= \frac { n }{ 2 } [2 × 1 + (n – 1) × 3] = \frac { n }{ 2 } (3n – 1)

Now,

S_1 + S_3 = \frac { n }{ 2 } (n + 1) + \frac { n }{ 2 } (3n – 1)

= 2n2 and S2 = n2

Hence S1 + S3 = 2S2

Concept: Sum of First n Terms of an AP
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