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10 cards from this deck
It means adding all terms in a sequence together.
un=a+(n−1)du_n = a + (n-1)dun=a+(n−1)d
Sn=n2[2a+(n−1)d]S_n = \frac{n}{2}[2a + (n-1)d]Sn=2n[2a+(n−1)d]
S3=1+7+13=21S_3 = 1 + 7 + 13 = 21S3=1+7+13=21
aaa is the first term; ddd is the common difference.
Rearrange to a=72−2da = 72 - 2da=72−2d, using known ddd value.
lll is the value at n=59n = 59n=59, which is 355355355.
It's an efficient way to sum with known first and last terms.
It is an arithmetic sequence.
Set up and solve the equation SN=2200S_N = 2200SN=2200.
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