Giải thích các bước giải:
a.Ta có $C$ nằm giữa cung $AB$
$\to \stackrel\frown{AC}=\stackrel\frown{CB}$
Mà $M\in \stackrel\frown{AC}, N\in \stackrel\frown{BC}, \stackrel\frown{CM}=\stackrel\frown{BN}$
$\to \stackrel\frown{AM}=\stackrel\frown{AC}-\stackrel\frown{CM}=\stackrel\frown{BC}-\stackrel\frown{BN}=\stackrel\frown{CN}$
$\to AM=CN$
b.Ta có:
$\stackrel\frown{MN}=\stackrel\frown{MC}+\stackrel\frown{CN}=\stackrel\frown{BN}+\stackrel\frown{CN}=\stackrel\frown{BC}=\stackrel\frown{AC}$
$\to MN=BC=AC$
