The formula to find a sum where you don't know the last number is S = 0.5n(2a+(n-1)d), where n is the amount of numbers, a is the first number, and d is the difference.
d in this case is 304-308 = -4, as it is the difference between each number.
So, putting this in the formula, you have S = 0.5(25){(2(308) + (24)(-4)}
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The sum of the arithmetic sequence
308, 304, 300, …, if there are 25 terms:
A sum of n first terms of arithmetic progression is calculated as:
Sn
= (a1 + an)/2*n
= (2a1 + d(n - 1)/2 * n.
S25
= (2*308 + (-4)(24))/2 * 25
= (616 - 96)/2 * 25
= 520/2 * 25
= 260 * 25
= 6,500
The formula to find a sum where you don't know the last number is S = 0.5n(2a+(n-1)d), where n is the amount of numbers, a is the first number, and d is the difference.
d in this case is 304-308 = -4, as it is the difference between each number.
So, putting this in the formula, you have S = 0.5(25){(2(308) + (24)(-4)}
So, S = 12.5(616 - 96) = 12.5(520) = 6500