\[\begin{array}{l}
20)\,\,\,Co\,\,\angle C = \angle K = {50^0}\\
\Rightarrow IK//BC.\\
Ap\,\,dung\,\,dinh\,\,ly\,\,Ta - let\,\,ta\,co:\\
\frac{x}{{10}} = \frac{8}{8} = 1 \Leftrightarrow x = 10.\\
21)\,\,C,\,\,D\,\,la\,\,trung\,\,diem\,\,cua\,\,OA,\,\,OB\\
\Rightarrow CD\,\,la\,duong\,\,trung\,binh\,\,cua\,\,\,\Delta OAB\\
\Rightarrow CD = \frac{1}{2}AB\\
\Leftrightarrow AB = 2CD = 2.3 = 6cm.
\end{array}\]
