$\displaystyle \begin{array}{{>{\displaystyle}l}} a.Theo\ ĐL\ sin:\\ \frac{a}{sin\ A} =\frac{b}{sin\ B} =\frac{c}{sin\ C} =2R\\ \Leftrightarrow \frac{12}{sin\ A} =\frac{b}{sin\ 70^{o}} =\frac{c}{sin\ 45^{o}} \ ( 1)\\ Trong\ \nabla ABC\ có\ A+B+C=180^{o}\\ \Rightarrow A=180^{o} -70^{0} -45^{o} =65^{o}\\ Từ\ ( 1) \Rightarrow b=\frac{12sin70^{o}}{sin65^{o}} \approx 12,442\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ c=\frac{12sin45^{o}}{sin65^{o}} \approx 9,3625\\ b.\ R=\frac{1}{2}\frac{12}{sin\ A} \approx 6,6203\\ c.\ S_{ABC} =\frac{1}{2} absinC\approx 52,7869\ \\ d.S_{ABC} =pr=\frac{a+b+c}{2} .r\\ \Leftrightarrow 52,7869=\frac{12+12,442+9,3625}{2} .r\\ \Rightarrow r=3,1231\\ \\ \end{array}$