Here is a more efficient method to solve 3a.
\text{If }T_n\text{ is the }n\text{-th Chebychev polynomial of the first kind so that}
T_n(\cos\theta)=\cos(n\theta)\text{ then using the }{}_2F_1\text{ hypergeometric we have that}
T_n(x)=\textstyle{}_2F_1(-n,n;\frac{1}{2};\frac{1-x}{2})
\text{and...