Sum of odd numbers between 1 and 1000 , which is divisible by 3=3+9+15+21+27+....+999=S (let) ∴ Let n be the number of terms in series and a is first term. ∴ l=a+(n−1)d, where l is last term and d is is common difference. 999=3+(n−1)×6 n−1=