a) Vì $\widehat{B_{1} }$`=`$75^o$

Mà $\widehat{ABC}$`+`$\widehat{B_{1} }$`=`$180^o$

`=>`$\widehat{ABC}$`=`$180^o$`-`$75^o$`=`$105^o$

b) Xét tứ giác `ABCD` có: 

 
$\widehat{CDA}$`+`$\widehat{BCD}$`+`$\widehat{ABC}$`+`$\widehat{DAB}$`=`$360^o$

Mà $\widehat{CDA}$`=`$75^o$ `;` $\widehat{BCD}$`=`$90^o$ `;` $\widehat{ABC}$`=`$105^o$

`=>`$\widehat{DAB}$`=`$360^o$`-`$105^o$`-`$90^o$`-`$75^o$`=`$90^o$

Lại có: $\widehat{DAB}$`+`$\widehat{A_{1}}$`=`$180^o$

`=>`$\widehat{A_{1}}$`=`$180^o$`-`$90^o$`=`$90^o$

$#chithanh17062010$