\(b,\)Xét \(\Delta MNA\) và \(\Delta MNB\) có :
\(\widehat{M}\) chung
\(\widehat{MNA}=\widehat{NBA}\) (cùng chắn \(\stackrel\frown{NA}\) )
\(\Rightarrow\Delta MNA\sim\Delta MNB\left(g-g\right)\)
\(\Rightarrow\dfrac{MN}{MA}=\dfrac{MB}{MN}\)
\(\Rightarrow MN^2=MA.MB\left(dpcm\right)\)







