\(M\left( {\dfrac{{ - {x_A} + {x_B}}}{2};\dfrac{{ - {y_A} + {y_B}}}{2};\dfrac{{ - {z_A} + {z_B}}}{2}} \right)\)

\(M\left( {\dfrac{{{x_A} + {x_B}}}{3};\dfrac{{{y_A} + {y_B}}}{3};\dfrac{{{z_A} + {z_B}}}{3}} \right)\)

\(M\left( {\dfrac{{{x_A} - {x_B}}}{2};\dfrac{{{y_A} - {y_B}}}{2};\dfrac{{{z_A} - {z_B}}}{2}} \right)\) 

\(M\left( {\dfrac{{{x_A} + {x_B}}}{2};\dfrac{{{y_A} + {y_B}}}{2};\dfrac{{{z_A} + {z_B}}}{2}} \right)\)