?$S_{△BEF}+S_{梯形ABDF}=S_{△ADE}$?
?$S_{△FCD}+S_{梯形ABDF}=S_{正方形ABCD}$?
?$S_{△BEF}-S_{△FCD}=80\ \mathrm {cm}^2$?
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。
