Ta có: $\widehat{ADC}$ - $\widehat{ABC}$ = $\widehat{B}$ - $\widehat{C}$
Do đó $\widehat{ADC}$ - $\widehat{ADB}$ = $20^o$
Suy ra $\widehat{ADB}$ = ( $180^o$ - $20^o$ ) : 2 = $80^o$
Mặt khác, $\widehat{HAD}$ = $90^o$ - $\widehat{ADH}$, suy ra $\widehat{HAD}$ = $10^o$