$\text{a,*Ta có:}$`\hat{BAC}`+`\hat{ABD}`=`180^@`$\text{(2 góc trong cùng phía)}$
Mà `\hat{BAC}`=`90^@`(giả thiết)=>`\hat{ABD}`=`180^@`-`90^@`=`90^@`
$\Rightarrow$`\hat{ABD}`=`90^@`
$\Rightarrow$b$\bot$AB
KL:Vậy:b$\bot$AB
$\text{b,*Ta có:}$`\hat{ACD}`+`\hat{BDC}`=`180^@`$\text{(2 góc trong cùng phía)}$
Mà `\hat{ACD}`=`120^@`(giả thiết)=>`\hat{BDC}`=`180^@`-`120^@`=`60^@`
$\Rightarrow$`\hat{BDC}`=`60^@`
$\text{KL:Vậy:}$`\hat{BDC}`=`60^@`
