In each part, find a formula for the general term of the sequence, starting with $n = 1$.

Enter the following information for $a_n=$.

(a) $$1,{\frac{1}{2}},{\frac{1}{4}},{\frac{1}{8}},...$$ $a_n=$

(b) $$1,-{\frac{1}{2}},{\frac{1}{4}},-{\frac{1}{8}},...$$ $a_n=$

(c) $${\frac{3}{4}},{\frac{15}{16}},{\frac{63}{64}},{\frac{255}{256}},...$$ $a_n=$

(d) $$0,\frac{1}{\sqrt{\pi}},\frac{4}{\sqrt[3]{\pi}},\frac{9}{\sqrt[4]{\pi}},...$$ $a_n=$