Giải thích các bước giải:
Ta có: $B, O, M$ thẳng hàng
$\to \dfrac{BC}{BN}.\dfrac{ON}{OA}.\dfrac{MA}{MC}=1$
$\to 3.\dfrac{ON}{OA}.3=1$
$\to \dfrac{ON}{OA}=\dfrac19$
$\to \dfrac{ON}{ON+AO}=\dfrac1{1+9}$
$\to\dfrac{ON}{AN}=\dfrac1{10}$
$\to \dfrac{S_{OBN}}{S_{ABN}}=\dfrac1{10}$
$\to S_{ABN}=10S_{OBN}$
$\to S_{ABN}=10$
$\to S_{ABC}=\dfrac{BC}{BN}S_{ABN}=3S_{ABN}=30$