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2D. Vẽ đường kính KD, KD cắt MN tại O'
\bigtriangleup KHM\sim \bigtriangleup KCD\Rightarrow \frac{KM}{KC}=\frac{KH}{KD}=\frac{R\sqrt{2}}{2R}=\frac{1}{\sqrt{2}}
\bigtriangleup KMN\sim \bigtriangleup KCB\Rightarrow \frac{KO'}{KH}=\frac{KM}{KC}=\frac{1}{\sqrt{2}}\Rightarrow KO'=R\Rightarrow O'\equiv O
