\[\begin{array}{l}
Dien\,\,\,tich\,\,hinh\,\,vuong\,\,\,ban\,\,dau:\,\,\,{\left( {a + b} \right)^2}.\\
Dien\,\,tich\,\,hinh\,\,\,vuong\,\,bi\,\,\,cat\,\,di:\,\,\,{\left( {a - b} \right)^2}.\\
\Rightarrow \,\,Dien\,\,tich\,\,phan\,\,\,con\,\,lai:\,\,\\
\,{\left( {a + b} \right)^2} - {\left( {a - b} \right)^2} = \left( {a + b - a + b} \right)\left( {a + b + a - b} \right) = 2b.2a = 4ab.
\end{array}\]
