Qua `O` ta kẻ `TI` đi qua `O` sao cho `TI////Cx` Ta có: `Cx////Dy` Mà `Cx////TI` `=>TI////Dy` `=>Cx////TI////Dy` Lại có: `Cx////TI` `=>hat{xCO}=hat{COI}=20^o(text{2 góc so le trong})` `TI////Dy` `=>hat{IOS}=hat{ODy}=30^o(text{2 góc so le trong})` Mà `hat{COD}=hat{COI}+hat{IOS}` mà `hat{COI}=20^o;hat{IOS}=30^o` `=>hat{COD}=20^o + 30^o` `=>hat{COD}=50^o` Vậy `hat{COD}=50^o`
