Question

find all the sum multiples of 7 lying between 100 and 1000.

Answer 1

Answer:

Step-by-step explanation:

the first no after 100 divisible by 7 is 105 and last before 1000 is 994.

thus, 

a = 105

l = 994

d = 7 (common difference)

n= ?

From 994, n can be obtained.

an = a + (n-1)d

994 = 105 + (n-1)7

994-105/7 = n-1

127 = n-1

n=128

Now Sum of 128 terms= n/2 (a + l)

=128/2 (105 + 994)

=64 x 1099

=70336

Thus, the answer is 70336.