Question

find the sum of first 20 natural numbers.

Answer 1

a = 1

d = 1

n= 20

Sn = n/2[ 2a +(n-1)d]

S20 = 20/2 [ 2*1 + 19*1]

S20 = 20/2 [ 2 + 19]

S20 = 10 * 21

S20 = 210

Required sum = 210

Answer 2

natural no.s are 
                         1,2,3,........20
d = a2-a1
   = 2-1
   = 1
a = 1
n = 20
              Sn = n/2(2a+(n-1)d)
              S20 = 20/2 (2*1 + (20-1)1)
              S20 = 10 (2+19)
                     = 10 (21)
                     = 210