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AMC / AIME · Arithmetic Sequences · Medium

Arithmetic Sequences: If the sum of the first math positive integers is math, find math.

Common differences.

Try the problem

If the sum of the first nn positive integers is 7878, find nn.

Hint and answer

Concept to use

Sums.

Hint

For sums, pair first and last terms or use Sn=n2(a1+an)S_n=\frac n2(a_1+a_n).

Answer

12

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