b, xét $ΔABC$ và $ΔHBA$:
$\widehat{A}=\widehat{H}=90$ độ
$\widehat{B}$ chung
$⇒ΔABC\simΔHBA$ (g-g)
$⇒\dfrac{AB}{HB}=\dfrac{BC}{AB}$
$⇔AB^2=BH.CB$
c, theo ông py-ta-go:
$BC^2=3^2+4^2=25$
$⇒BC=5$ cm
$⇒BH=AB^2:BC=9:5=1,8$ cm
$\dfrac{AB}{AC}=\dfrac{HB}{HA}$
$⇒AB.AH=AC.HB$
$⇒AH=\dfrac{AC.HB}{AB}=2,4$ cm
sorry cậu mk kb làm ý a