Đáp án: $100cm^2$
Giải thích các bước giải:
Ta có $AB//CD\to \widehat{OAB}=\widehat{OCD},\widehat{OBA}=\widehat{ODC}$
$\to\Delta OAB\sim\Delta OCD(g.g)$
$\to \dfrac{S_{OAB}}{S_{OCD}}=(\dfrac{OA}{OC})^2$
$\to (\dfrac{OA}{OC})^2 =\dfrac9{49}$
$\to \dfrac{OA}{OC}=\dfrac37$
$\to \dfrac{S_{OAD}}{S_{OCD}}=\dfrac{OA}{OC}=\dfrac37$
$\to S_{OAD}=\dfrac37\cdot S_{OCD}=21$
Lại có : $\dfrac{S_{OAB}}{S_{OBC}}=\dfrac{OA}{OC}=\dfrac27$
$\to S_{OBC}=\dfrac73S_{OAB}=21$
$\to S_{ABCD}=S_{OAB}+S_{OBC}+S_{OCD}+S_{ODA}=100$
