18. As shown in the figure, it is formed by two squares with side lengths of 5 cm and 4 cm respectively. What is the area of the shaded part in square centimeters?
【Analysis】In the figure below, the shaded part is a model of an hourglass, it is known that $H G: G C=A H: C D=5: 4$, and from $H G+G C=5$, we can find that $G C=5 \times \frac{4}{4+5}=\frac{20}{9}$, then $F G=4-\frac{20}{9}=\frac{16}{9}, S_{\triangle D F G}=\frac{1}{2} \times \frac{16}{9} \times 4=\frac{32}{9}$ square centimeters.