Answer:
Sₙ = 57/2 * (504 + 896) = 39900
Step-by-step 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
