Chapter 4 - Polynomials - 4.5 Multiplication of Polynomials - 4.5 Exercise Set - Page 268: 21

$-\dfrac{1}{20}x^{12}$

The product of the given expression, $ \left( -\dfrac{1}{4}x^4 \right)\left( \dfrac{1}{5}x^8 \right) $, is \begin{array}{l}\require{cancel} \left( -\dfrac{1}{4} \right)\left( \dfrac{1}{5} \right)x^{4+8} \\\\= -\dfrac{1}{20}x^{12} .\end{array}