Giải thích các bước giải:
a.Ta có $AB//CD\to S_{ACD}=S_{BCD}$
$\to S_{ACD}-S_{OCD}=S_{BCD}-S_{OCD}$
$\to S_{OAD}=S_{OBC}$
b.Ta có $AB//EG//CD$
$\to \dfrac{EO}{AB}=\dfrac{DO}{DB}=\dfrac{CO}{CA}=\dfrac{OG}{AB}$
$\to OE=OG$
c.Ta có $\widehat{AOB}=\widehat{COD},\widehat{OAB}=\widehat{ODC}$ vì $AB//CD$
$\to \Delta OAB\sim\Delta OCD(g.g)$
$\to \dfrac{S_{OAB}}{S_{OCD}}=(\dfrac{OA}{OC})^2$
$\to (\dfrac{OA}{OC})^2=\dfrac{9}{16}$
$\to\dfrac{OA}{OC}=\dfrac34$
$\to \dfrac{S_{AOD}}{S_{OCD}}=\dfrac34$
$\to S_{AOD}=\dfrac34S_{OCD}=12$
$\to S_{BOC}=S_{AOD}=12$
$\to S_{ABCD}=S_{AOB}+S_{AOD}+S_{OCD}+S_{OBC}=49$