题目内容
如图所示,等腰直角三角形ABC中,∠A=90°,BC=
,DA⊥AC,DA⊥AB,若DA=1,且E为DA的中点.求异面直线BE与CD所成角的余弦值.


异面直线BE与CD所成角的余弦值为

取AC的中点F,连接EF,BF,在△ACD中,E、F分别是AD、AC的中点,

∴EF∥CD,
∴∠BEF即为异面直线BE与CD所成的角或其补角.
在Rt△EAB中,AB=AC=1,
AE=
AD=
,∴BE=
,
在Rt△EAF中,
AF=
AC=
,AE=
,∴EF=
,
在Rt△BAF中,AB=1,AF=
,∴BF=
,
在等腰三角形EBF中,
cos∠FEB=
=
,
∴异面直线BE与CD所成角的余弦值为
.

∴EF∥CD,
∴∠BEF即为异面直线BE与CD所成的角或其补角.
在Rt△EAB中,AB=AC=1,
AE=



在Rt△EAF中,
AF=




在Rt△BAF中,AB=1,AF=


在等腰三角形EBF中,
cos∠FEB=


∴异面直线BE与CD所成角的余弦值为


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