Question

Find the sum of all 2 digits natural no. Divisible by7

Answer 1

First two digit no. divisible by 7 = 14
Last two digit no. divisible by 7 = 98
So , first term of A.P. (a)= 14
Last term of A.P. (l)= 98
Common difference (d) = 7
98 = 14 + (n-1)7
n = 13
Sum = n/2 (a+l) = 13/2 (112) = 728

Answer 2

Hi !

The 2 digits natural no: divisible by 7 forms an A.P :-

14, 21 , 28  ......... 98

a = 14
d = 7
l = 98

l = a + (n-1)d
98 = 14 + (n-1)7
84 =  (n-1)7
(n-1) = 12

n = 13

Sn = n/2( a + l)
S₁₃ = 13/2 ( 14 + 98)
      = 13/2 * 112
      = 728

sum of 2 digit no: divisible by 7 = 728