so you want the sum of
(1002+1012+1022+...+2992) + (1009+1019+...+1999)
two AP's , one with a = 1002, d=10, the other with a=1009, d=10
How many terms in each one?
First: term(n) = a + (n-1)d
2992 = 1002 + 10(n-1)
1990 = 10n - 10
n = 200
Find the number of terms in the second series (the number of terms is not 200).
Find the sum of each series using your favourite formula, then add the two sums.
What is the sum of all numbers ending in 2 or 9 between 1000 and 2000?
Answer is 300 100
2 answers
Makes sense. Thank you