6.如图,在正方体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,截面A1BDA_{1}BDA1BD与底面ABCDABCDABCD所成锐二面角A1−BD−AA_{1}-BD-AA1−BD−A的正切值为((( )))