Question

Find the sum of all multiples of 7 lying between
500 & 900​

Answer 1

Sₙ = 57/2 * (504 + 896) = 39900

Explanation:

first multiple of 7 after 500 is: 72 *7 = 504

last multiple of 7 before 900 is: 128 *7 = 896

a₁ = 504

aₙ = 896

where n = 128-72 +1 = 57

sum of arithmetic sequence: Sₙ = n/2 * (a₁ + aₙ)

Sₙ = 57/2 * (504 + 896) = 39900