Here is a "proof" or verification for 3 terms only but it holds true for any n terms:
Expand the following:
binomial(n, 1) + 2 binomial(n, 2) + 3 binomial(n, 3) = n×2^(n - 1)
n (n - 1) = n n - n:
binomial(n, 1) + n n - n + 3 binomial(n, 3) = n×2^(n - 1)
n n = n^2:
binomial(n, 1) - n + n^2 + 3 binomial(n, 3) = n×2^(n - 1)
(n - 2) (n - 1) = (n) (n) + (n) (-1) + (-2) (n) + (-2) (-1) = n^2 - n - 2 n + 2 = n^2 - 3 n + 2:
binomial(n, 1) - n + n^2 + (n^2 - 3 n + 2 n)/2 = n×2^(n - 1)
n/2 (n^2 - 3 n + 2) = (n n^2)/2 + 1/2 n (-3 n) + (n 2)/2:
(n n^2)/2 - (3 n n)/2 + (2 n)/2 + n^2 + binomial(n, 1) - n = n×2^(n - 1)
(n (-3) n)/2 = (-3 n^2)/2:
(n n^2)/2 + n^2 + (-3 n^2)/2 + (2 n)/2 + binomial(n, 1) - n = n×2^(n - 1)
(n n^2)/2 = n^(1 + 2)/2:
n^(1 + 2)/2 + n^2 - (3 n^2)/2 + (2 n)/2 + binomial(n, 1) - n = n×2^(n - 1)
1 + 2 = 3:
n^3/2 + n^2 - (3 n^2)/2 + (2 n)/2 + binomial(n, 1) - n = n×2^(n - 1)
Grouping like terms, n^3/2 + n^2 + 1/2 n^2 (-3) + (n 2)/2 + binomial(n, 1) + n (-1) = n^3/2 + (n^2 - (3 n^2)/2) + (n - n + n):
n^3/2 + (n^2 - (3 n^2)/2) + (n - n + n) = n×2^(n - 1)
n^2 - (3 n^2)/2 = -n^2/2:
n^3/2 + -(n^2)/2 + (n - n + n) = n×2^(n - 1)
n - n + n = n:
n^3/2 - n^2/2 + n = n×2^(n - 1) Courtesy of Mathematica 11 Home Edition.