Question

Find the sum if all three digit natural numbers which are divisible by 7

Answer 1

105,112,119.......994
a= 105
d= 7
an=994
 first we want to find no.of natural numbers
  so, 
      an=a+[n-1]d
     994=105+[n-1]7
     994=105+7n-7
     994=98+7n
     7n=994-98
     7n=896
   n=896/7 = 128
Sn= n/2[a+an]
Sn=128/2[105+994]
Sn=64[1099]
Sn=70336

Answer 2

Sn=70336

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