Question

find the sum of all 3 digit natural number which is multiple of 7

Answer 1

3 digit natural numbers which are multiple of 7 are = 105, 112, 119.....994

This series is an AP.

First term (a) = 105

Common difference (d) = 7

Now,

an = a + (n - 1)d

=> 994 = 105 + (n - 1)7

=> 994 - 105 = 7n - 7

=> 889 + 7 = 7n

=> n = 896/7 = 128

Then,

Sum of the terms = n/2 × (a + l)

= 128/2 × (105 + 994)

= 64 × 1099

= 70336

Answer 2

Step-by-step explanation:

3 digits natural no which r multiple of 7 is 105,112,119........994

This series is an Ap

First term is a =105

Second term is d =7

Now,

An =a+(n-1)d

994=105+(n-1)7

944-105=7n-7

889+7=7n

n=896/7=128.

Then,

Sum of the terms =n/2 multiply (a+1)

128/2 multiply 105+994

64 multiply 1099

70336