d. △ACF vuông tại E có \(\widehat{FAC}=60^0\Rightarrow AF=\dfrac{AC}{2}\)
\(AE.AC=AF.AB\Rightarrow\dfrac{AE}{AB}=\dfrac{AF}{AC}\Rightarrow\)△AEF∼△ABC (c-g-c)
\(\Rightarrow\dfrac{S_{AEF}}{S_{ABC}}=\left(\dfrac{AF}{AC}\right)^2=\left(\dfrac{1}{2}\right)^2=\dfrac{1}{4}\)
\(\Rightarrow S_{AEF}=\dfrac{1}{4}S_{ABC}=\dfrac{1}{4}\left(đvdt\right)\)
\(\Rightarrow S_{BCEF}=S_{ABC}-S_{AEF}=1-\dfrac{1}{4}=\dfrac{3}{4}\left(đvdt\right)\)
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