a/ Xét $\Delta ABI$ và $\Delta HCI$:
$\widehat{IAB}=\widehat{IHC}(=90°)$
$\widehat{AIB}=\widehat{HIC}$ (đối đỉnh)
$\to\Delta ABI\backsim\Delta HCI(g-g)$
$\\$
b/ $\Delta ABI\backsim \Delta HCI$
$\to \widehat{IBA}=\widehat{ICH}$
Mà $\widehat{IBA}=\widehat{IBC}$
$\to\widehat{IBC}=\widehat{ICH}$