Giải thích các bước giải:

a.Xét $\Delta MAB,\Delta MCN$ có:

$MA=MC$

$\widehat{AMB}=\widehat{CMN}$

$MB=MN$

$\to \Delta MAB=\Delta MCN(c.g.c)$

$\to \widehat{MCN}=\widehat{MAB}=90^o$

$\to AC\perp CN$

b.Xét $\Delta MAN,\Delta MCB$ có:

$MA=MC$

$\widehat{AMN}=\widehat{BMC}$

$MN=MB$

$\to \Delta AMN=\Delta CMB(c.g.c)$

$\to AN=BC,\widehat{MAN}=\widehat{MCB}$

$\to AN//BC$