Solution
= Solution
From the exponential definition, $\cos(iu)=\cosh u$. Therefore
$$
\boxed{\cos(2i\ln2)=\cosh(\ln4)=\frac{4+1/4}{2}=\frac{17}{8}.}
$$
A cosine of a nonreal argument need not lie between $-1$ and $1$; that bound applies to real arguments.