Question

find the sum of all natural numbers between 1 to 1000 which are divisible by 7?​

Answer 1

Let,

a= first term

d=common difference

a=7,d=7

tn = 994

$tn = a + (n - 1)d$

994 = 7 + (n - 1)7

994-7 = 7n - 7

987 + 7 = 7n

994/7 = n

n = 142

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

S142 = 142/2 [2*7 + 141*7]

S142 = 71 ( 14 + 987)

S142 = 71 * 1001

S142 = 71071

Answer:

The sum of natural no.s between 1 to 1000 which are divisible by 7 is 71071