Giải thích các bước giải:

a.Xét $\Delta ABM,\Delta ACN$ có:

Chung $\hat A$

$\widehat{AMB}=\widehat{ANC}(=90^o)$

$\to \Delta AMB\sim\Delta ANC(g.g)$

$\to \dfrac{AM}{AN}=\dfrac{AB}{AC}$

$\to AM.AC=AN.AB$

b.Xét $\Delta AMN,\Delta ABC$ có:

Chung $\hat A$

$ \dfrac{AM}{AN}=\dfrac{AB}{AC}$

$\to \Delta AMN\sim\Delta ABC(c.g.c)$

$\to \widehat{ANM}=\widehat{ACB}$

c.Từ b $\to \widehat{ANM}=\widehat{ACB}$

$\to \widehat{BAQ}=\widehat{NAK}=90^o-\widehat{ANK}=90^o-\widehat{ACB}=90^o-\widehat{ACD}=\widehat{DAC}=\widehat{CAH}$