20. (Total: 14 points; Part 1: 7 points; Part 2: 7 points)\\
$ABCD - A_1B_1C_1D_1$ is a right square prism with base edge length 1 and height $AA_1 = 2$. Find:\\
(1) The angle between skew lines $BD$ and $AB_1$ (express the result using inverse trigonometric functions);\\
(2) The volume of tetrahedron $AB_1D_1C$
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