a,[tex]\frac{-1}{2}=\frac{a}{-10}=\frac{-7}{b}=\frac{c}{-20}[/tex]
[tex]\Rightarrow \left\{\begin{matrix} \frac{a}{-10}=\frac{-1}{2} & & \\ \frac{-7}{b}=\frac{-1}{2} & & \\ \frac{c}{-20}=\frac{-1}{2} & & \end{matrix}\right.[/tex]
=>.....
b,Tương tự nhé!