Câu 21: Hpt: [tex]\left\{\begin{matrix} T_A+T_B=mg\\ T_A.0,3=T_B.0,7 \end{matrix}\right.\Rightarrow T_A,T_B=...[/tex]
Câu 22: [tex]\left\{\begin{matrix} MC+NC=1,2\\ MC=\frac{2}{3}NC \end{matrix}\right.\Rightarrow MC,NC=...[/tex]
Hpt [tex]\left\{\begin{matrix} T_A+T_B=(m+m')g\\ T_A.\frac{MN}{2}+m'g(\frac{MN}{2}-MC)=T_B.\frac{MN}{2}\Rightarrow T_A,T_B=... \end{matrix}\right.[/tex]