Answer
$t^4-0.04$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $
(x^4+0.1)(x^4-0.1)
$, is equivalent to
\begin{array}{l}\require{cancel}
(t^2)^2-(0.2)^2
\\\\=
t^4-0.04
.\end{array}