Cho

- Hướng dẫn giải

a) $\Delta$ABM = $\Delta$ABD (c.c.c)

Suy ra $\widehat{BAM}=\widehat{CAM}$ Suy ra đpcm

b) Suy ra $\widehat{AMB}=\widehat{AMC}$ ( Góc tương ứng)

Mà $\widehat{AMB}+\widehat{AMC}$ = 180°

$=> \widehat{AMB}=\widehat{AMC} ={90}^0$. Suy ra $AM \perp BC$