Question 22
A real-valued function $f$ has the property that for all real numbers $a$ and $b,$
$$
\begin{gathered} f(a+b)+f(a-b) \\ =2f(a)f(b). \end{gathered}
$$
Which one of the following cannot be the value of $f(1)?$
$$
\begin{gathered} f(a+b)+f(a-b) \\ =2f(a)f(b). \end{gathered}
$$
Which one of the following cannot be the value of $f(1)?$