\[1)\ A(3)\ и\ B(7):\]
\[L = |3 - 7| = | - 4| = 4.\]
\[2)\ A( - 2)\ и\ B(4):\]
\[L = | - 2 - 4| = | - 6| = 6.\]
\[3)\ A( - 2)\ и\ B( - 6):\]
\[L = | - 2 + 6| = |4| = 4.\]
\[4)\ A(a)\ и\ B(b):\]
\[L = |a - b|.\]