Let $f(t)=\cos ^{2} t+\cos ^{2}\left(t+\frac{2 \pi}{3}\right)+\cos ^{2}\left(t-\frac{2 \pi}{3}\right)$
(a) Complete the table. (Use a calculator.)
$$\begin{array}{lllll}
\hline t & 1 & 2 & 3 & 4 \\
\hline f(t) & & & & \\
\hline
\end{array}$$
(b) On the basis of your results in part (a), make a conjecture about the function $f .$ Prove that your conjecture is correct.