a/ Ta có: $\widehat{BAC}$ = $\widehat{ACD}$ = $35^{o}$
mà hai góc này so le trong
⇒ AB // DC (dhnb)
b/ Ta có: AB // DC (c.m.t) ⇒ $\widehat{D}$ + $\widehat{DAB}$ = $180^{o}$ (Trong cùng phía)
$80^{o}$ + $\widehat{DAB}$ = $180^{o}$
$\widehat{DAB}$ = $180^{o}$ - $80^{o}$ = $100^{o}$
Vì AC nằm trong $\widehat{DAB}$:
⇒ $\widehat{DAC}$ + $\widehat{CAB}$ = $\widehat{DAB}$
⇒ $\widehat{DAC}$ + $35^{o}$ = $100^{o}$
⇒ $\widehat{DAC}$ = $100^{o}$ - $35^{o}$ = $65^{o}$