Sum of n numbers
Algebra
Level
2
Find the sum of all numbers starting between 19 and 79 both included. i.e Determine S where S = 19 + 20 + 21 + 22 + 23 + 24 + 25.........79.
2910
2989
2970
3160
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1 solution
Sum to n terms of a series is given by s = n * (n + 1)/2 So the sum of all numbers between 19 and 79 both inclusive can be written as (Sum of all numbers from 1 to 79) - (Sum of all numbers from 1 to 18)
S = 79* 80/2 -18 * 19/2 S = 2989