Question

find the sum of all odd integers between 78 to 500 which are divisible by 7​

Answer 1

Given: The numbers from 78 to 500 which are divisible by 7​

To find: The sum of them.

Solution:

• Now the fist term is 91, last term will be 497.

• The common difference will be 14 as the difference between odd multiple of 7 is 14.

                   a(n) = a + (n-1)d

                   497 = 91 + (n-1)14

                   406 / 14 = n-1

                   29 + 1 = n

                   30 = n

• Now the formula for sum is :

                   S(n) = n/2(a + l)

• where S(n) is sum, a is first term, n is number of terms and l is last term

                   S(n) = 30/2(91 + 497)

                   S(n) = 15(588)

                   S(n) = 8820

Answer:

              So the sum of all odd integers between 78 to 500 which are divisible by 7​ is 8820.