#### Answer

$5$

#### Work Step by Step

Using $(a+b)(a-b)=a^2-b^2$, or the product of the sum and difference of like terms, and using the properties of radicals, the given expression, $
(1+2i)(1-2i)
,$ simplifies to
\begin{array}{l}\require{cancel}
(1)^2-(2i)^2
\\\\=
1-4i^2
\\\\=
1-4(-1)
\\\\=
1+4
\\\\=
5
.\end{array}