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Arithmetic Sequences
hard
0:00.0
If
a
1
=
1
,
d
=
2
a_1=1, d=2
a
1
=
1
,
d
=
2
, find
S
n
S_n
S
n
.
A
n
2
n^2
n
2
B
n
(
n
+
1
)
/
2
n(n+1)/2
n
(
n
+
1
)
/2
C
2
n
−
1
2n-1
2
n
−
1
D
n
n
n
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