<image>As shown in the , in the cube ABCD-A_{1}B_{1}C_{1}D_{1}, E and F are the midpoints of B_{1}C_{1} and CC_{1}, respectively.
Find: 
(1) the angle formed by AB and DD_{1};
(2) the angle formed by AC and B_{1}D_{1};
(3) the angle formed by AC and BC_{1};
(4) the angle formed by A_{1}D and EF.