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Mathematics
Problem
∑
r
=
1
n
∑
p
=
0
r
−
1
n
C
r
⋅
r
C
p
⋅
2
p
\sum_{r=1}^{n} \sum_{p=0}^{r-1} {}^n C_r \cdot {}^r C_p \cdot 2^p
∑
r
=
1
n
∑
p
=
0
r
−
1
n
C
r
⋅
r
C
p
⋅
2
p
is equal to a.
4
n
−
3
n
+
1
4^n - 3^n + 1
4
n
−
3
n
+
1
b.
4
n
−
3
n
−
1
4^n - 3^n - 1
4
n
−
3
n
−
1
c.
4
n
−
3
n
+
2
4^n - 3^n + 2
4
n
−
3
n
+
2
d.
4
n
−
3
n
4^n - 3^n
4
n
−
3
n
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