Suppose that an 20 and sn 60 Find nSolutionThe sum Sn of f

Suppose that a_n = 20 and s_n = 60. Find n.

Solution

The sum Sn of first n terms of an arithmetic series with first term a1 and n term a_n is

S_n=n/2(a+a_n)

We know that

S_n=n/2(a+a_n)

Let a=0

Then 60.2= n.20

N=6.