$?S_{△BEF}+S_{梯形ABDF}=S_{△ADE}?$
$?S_{△FCD}+S_{梯形ABDF}=S_{正方形ABCD}?$
$?S_{△BEF}-S_{△FCD}=80\ \mathrm {cm}^2?$
$則?S_{△ADE}-S_{正方形ABCD}=80\ \mathrm {cm}^2?$
$?S_{△ADE}=S_{正方形ABCD}+80=20×20+80=480(\mathrm {cm}^2)?$
$?480×2÷20=48(\mathrm {cm})?$
$?48-20=28(\mathrm {cm})?$
$答：線段?BE?的長為?28?厘米?.?$
