$\displaystyle \begin{array}{{>{\displaystyle}l}} \mathrm{a.\ Do\ \widehat{COA} < \widehat{COD} \ \Rightarrow OC\ nằm\ giữa\ OA\ và\ OD}\\ \mathrm{\ \ \ \ \ \ \widehat{COD} < \widehat{BOD} \ \Rightarrow OD\ nằm\ giữa\ OC\ và\ OB}\\ \mathrm{Ta\ có:\ \ \widehat{COA} +\ \widehat{COD} +\ \widehat{DOB} \ =180^{o}}\\ \mathrm{\Leftrightarrow a+a+10+a+20=180^{o}}\\ \mathrm{\Leftrightarrow a=50^{o}}\\ \mathrm{b.\ Trên\ cùng\ một\ nửa\ mặt\ phẳng\ bở\ AB,\ có:}\\ \mathrm{\widehat{xOA} +\ \widehat{xOy} +\ \widehat{yOB} \ =180^{o}}\\ \mathrm{\Rightarrow \widehat{xOy} =180^{o} -22^{o} -48^{o} =110^{o}}\\ \mathrm{c.\widehat{xOD} \ và\ \ \widehat{xOE} \ là\ hai\ góc\ kề\ bù.}\\ \mathrm{\Leftrightarrow \widehat{xOD} \ +\ \widehat{xOE} =180^{o}}\\ \mathrm{\Rightarrow \widehat{xOE} =180^{o} -a-10^{o} =170^{o} -a} \end{array}$