Find the sum of all integers between 1200 and 2900, inclusive, which are not divisible by 9.

Answer 1

9*134 = 1206
9*322 = 2898

So, subtract all the multiples of 9 from the total sum:

∑[k=1200,2900] k - ∑[k=134,322] 9k
= 3 487 050 - 387 828
= 3 099 222