Ta có \(\widehat{MAC}=\dfrac{1}{2}sđ\stackrel\frown{MB}=\dfrac{1}{2}sđ\stackrel\frown{MA}=\widehat{MDA}\Rightarrow\Delta MAC\sim\Delta MDA\left(g.g\right)\Rightarrow MA^2=MC.MD=\left(12\right)^2\Rightarrow MA=12\).
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