Question

The sum of the 3rd and 11th term of an Ap is 34. find the sum of its 13 terms

Answer 1

We know that
third term = a+2d-------a
and,
11th term = a+ 10d--------b
now ,
a+b=2a+12d=34----------- [given]
now sum of 13 terms is
Sn=S13 = 13/2×[2a+12d]
S13=13/2×34 ---------[2a+12d=34]

therefore S13= 13 ×17= 221
answer=221