Question

Find the sum of n terms of the A.P. whose kth terms is 5k + 1

Answer 1

Given an AP whose kth term is 5k + 1 

To find : the sum of n terms of an AP whose kth term is 5k + 1 

So substituting k = 1 to get the first term a₁ = 6 

Substituting k = 2 to get the second term a₂ = 11 

d = a₂ - a₁ = 5 

We know that the sum of AP is given by the formula :

s = \(\frac{n}{2}\)(2a + (n-1)d)

Substituting the values in the above equation,

s = \(\frac{n}{2}\)(12 + (n-1)5)

s = \(\frac{5n^2+7n}{2}\)