Đáp án:
Giải thích các bước giải:
\[\begin{array}{l}
{S_{ABC}} = \frac{1}{2}AB.AC.\sin \widehat A\\
{S_{ADE}} = \frac{1}{2}AD.AE.\sin \widehat A\\
AD = AB\cos \widehat A;AE = AC.\cos \widehat A = > {S_{ADE}} = \frac{1}{2}AB.AC.\sin \widehat A.{\cos ^2}\hat A = {S_{ABC}}.{\cos ^2}\widehat A\\
{S_{BEDC}} = {S_{ABC}} - {S_{ADE}} = {S_{ABC}}(1 - {\cos ^2}\widehat A) = {S_{ABC}}.{\sin ^2}\widehat A
\end{array}\]