Find the closed form the following

a) n
sigma (-1)^i * (2i - 1)^2
i=0

b) n
sigma (n choose i) * (1/2^i)
i=0

c) n
sigma (2n choose 2i) * (1/2^i)
i=0

d) n
sigma (n + i choose i) * (1/2^i)
i=0