\(\widehat{AOM}=\widehat{BCM}\) (cùng bù \(\widehat{BOM}\))
Mà \(\left\{{}\begin{matrix}\widehat{AOD}=\dfrac{1}{2}\widehat{AOM}\\\widehat{BCO}=\dfrac{1}{2}\widehat{BCM}\end{matrix}\right.\) \(\Rightarrow\widehat{AOD}=\widehat{BCO}\)
\(\Rightarrow\Delta_VAOD\sim\Delta_VBCO\) \(\left(g-g\right)\)
\(\Rightarrow\dfrac{AO}{BC}=\dfrac{AD}{OB}\Rightarrow OA.OB=AD.BC\Rightarrow R^2=AD.BC\)
\(\Rightarrow R=2\sqrt{10}\) \(\Rightarrow AB=2R=4\sqrt{10}\)
\(S=\dfrac{1}{2}\left(AD+BC\right).AB-\dfrac{1}{2}.\pi R^2\approx25,7\left(cm^2\right)\)
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