A:1∶7
B:1∶5
C:1∶9
D:2∶9
如图,在三棱柱ABC﹣A1B1C1中,四边形A1ABB1为菱形,∠A1AB=45°,四边形BCC1B1为矩形,若AC=5,AB=4,BC=3
(1)求证:AB1⊥面A1BC;(2)求二面角C﹣AA1﹣B的余弦值.
Mr. Black has a daughter. Tod ay is her ____ (twelve) birthday.
如图所示,请你作出图中足球所受重力的示意图.
为测量如图所示上山坡道的倾斜度,小明测得图中所示的数据,则该坡道倾斜角α的正切值是( )