\[\boxed{\mathbf{738.}ОК\ ГДЗ\ –\ домашка\ на\ 5}\]
\[На\ всей\ плоскости\ \text{Oxy\ }\]
\[значение\ z = 0.\]
\[У\ середины\ всех\ указанных\ \]
\[отрезков\ z = 0.\]
\[\textbf{а)}\ A(2;3; - 1);B(5;7;k):\]
\[k - 1 = 0\]
\[k = 1.\]
\[\textbf{б)}\ A(0;4;k);B(3; - 8;2):\]
\[k + 2 = 0\]
\[k = - 2.\]
\[\textbf{в)}\ A(5;3;k);B(3; - 5;3k):\]
\[k = 3k\]
\[2k = 0\]
\[k = 0.\]