Question

Find S2 for the following given A.P. 3,5,7,9... .

Answer 1

A1 = 3, A2 = 5
S2 = A1+A2 = 3+5 = 8

Answer 2

It can be solved by the formula of ' sum of nth term "
It says 
              Sn=n/2[2a +(n-1)d],    where 'n' is the 'no. of terms' and 'a' is the first term of the given A.P and 'd' is the common difference between first term and second term ( a2-a1 ).


Sol :- 'a1' = 3 and 'd' = a2-a1 = 5-3 = 2 and 'n' is 2 given

NOW,  Sn = n/2 [2a + (n-1)d]
             S2 = 2/2 [2(3) + (2-1)2]
                    =1 [6+2]
                     =8.