在线解题过程见链接。https://www.geogebra.org/m/ejxbtdyg
\(解：\)
\(过E作AD垂线，交AD于点E’\)
\(DE’即为ΔCDE的高。\)
\(∵大正方形面积是81cm^2,∴AD=CD=9\)
\(∵ΔDEE’∼DAE\)
\(∴\frac{E’D}{ED}=\frac{ED}{AD}=\frac{E’D}{4}=\frac{4}{9}\)
\(∴E’D=\frac{16}{9}\)
\(∴ΔCDE面积=\frac{1}{2} \cdot \frac{16}{9}\cdot 9=8cm^2\)