Vi hai tia OB, OC cùng nửa mặt phẳng bờ chứa tia OA sao cho \(\widehat{BOA}\)= $145^0$ , \(\widehat{COA}\)= $55^0$ (gt) 

=> \(\widehat{COA}\) < \(\widehat{BOA}\) 

=>  tia $OC$ nằm giữa hai tia $OA,OB$.

=>  \(\widehat{COA}\)  + \(\widehat{COB}\) = \(\widehat{BOA}\)   

=>  $55^0$  +  \(\widehat{COB}\) = $145^0$         

=>  \(\widehat{COB}\) = $145^0-55^0=90^0$ 

Vậy  \(\widehat{COB}\)= $90^0$