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10 cards from this deck
Adding all terms in a sequence together
un=a+(n−1)du_n = a + (n-1)dun=a+(n−1)d
Sum of first nnn terms of a series
Sn=n2[2a+(n−1)d]S_n = \frac{n}{2}[2a + (n-1)d]Sn=2n[2a+(n−1)d]
aaa=first term, ddd=common diff, nnn=position of last term
Sn=n2[a+l]S_n = \frac{n}{2}[a + l]Sn=2n[a+l] where lll is last term
Using S59−S21S_{59} - S_{21}S59−S21 (removes needed u21u_{21}u21)
S59−S20S_{59} - S_{20}S59−S20 to keep u21u_{21}u21
q−p+1q - p + 1q−p+1 (add 1 to count first term)
More efficient for algebraic work when first & last terms known
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