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12 cards from this deck
A sequence summed up
An arithmetic sequence summed up
Sn=n2[2a+(n−1)d]S_n=\frac{n}{2}[2a+(n-1)d]Sn=2n[2a+(n−1)d]
Sum of first nnn terms: Sn=T1+T2+...+TnS_n=T_1+T_2+...+T_nSn=T1+T2+...+Tn
First term of sequence
Common difference
Find first few terms, identify by observation
Rearrange TnT_nTn and equate with given term
Tn=dn+(a−d)T_n=dn+(a-d)Tn=dn+(a−d)
a=−4a=-4a=−4
d=2d=2d=2
500500500
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