Solution
= Solution
Parseval's identity for the cosine <series>, with $\|T_0\|_w^2=\pi$ and $\|T_k\|_w^2=\pi/2$ for $k\ge1$, gives the claimed equality.
= Solution
Parseval's identity for the cosine <series>, with $\|T_0\|_w^2=\pi$ and $\|T_k\|_w^2=\pi/2$ for $k\ge1$, gives the claimed equality.