a) a_{n}= frac{1}{2^{n-1}}

S_{n}=1+frac{1}{2}+frac{1}{2^{2}}+...+frac{1}{2^{n-1}} = frac{1}{1-frac{1}{2}}=2

b) p_{n}=4.frac{1}{left (sqrt{2}  right )^{n-1}}