Question

how do you add up 31 to 40

Answer 1

Use this fomula n(n+1)/2

Answer 2

sorry (40-31+1)*(31+40)/2

Answer 3

Find the sum of 1 to 40 then subtract it by the sum of 1 to 30. this should leave you the sum of 31 to 40.

Answer 4

actually. you can apply gauss series.
1.general formula n*(n+1)/2
2.when we need to find the sum from a to b,:
first, find the no.of integers from a to b. -> a+b-1
second, use (a+b-1)*(a+b)/2 will give you the answer.