Vì AB//CD
nên \(\dfrac{OA}{OC}=\dfrac{OB}{OD}=\dfrac{AB}{CD}=\dfrac{1}{3}\)
Vì OA/OC=1/3
nên \(S_{AOB}=\dfrac{1}{3}\cdot S_{BOC}\)
=>\(S_{BOC}=4\cdot3=12\left(cm^2\right)\)
Vì \(OB=\dfrac{1}{3}OD\)
nên \(S_{AOB}=\dfrac{1}{3}\cdot S_{AOD}\)
=>\(S_{AOD}=4\cdot3=12\left(cm^2\right)\)
Vì \(AO=\dfrac{1}{3}OC\)
nên \(S_{AOD}=\dfrac{1}{3}\cdot S_{DOC}\)
=>\(S_{DOC}=12\cdot3=36\left(cm^2\right)\)
\(S_{ABCD}=S_{AOB}+S_{BOC}+S_{COD}+S_{AOD}\)
\(=4+12+12+36=16+12+36\)
=28+36
=64cm2