Question

The sum of first 21 terms of the AP : 5, 13, 21, ..... is equal to

Answer 1

a= 5

d= 13-5= 8

n=21

by the formula

s21=21/2(10+20x8)

s21= 21/2(170)

s21=3570/2

s21=1785

Therefore sum of first 21 terms are 1785

Answer 2

hey friend 1785 is ur answer...

pls refer my attachment:

hope it helps....