如图所示,在长方体ABCD—A 1 B 1 C 1 D 1 中,AB=BC=1,BB 1 =2,
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如图所示,在长方体ABCD—A 1 B 1 C 1 D 1 中,AB=BC=1,BB 1 =2, ![]() E是棱CC 1 上的点,且CE= ![]() (1)求三棱锥C—BED的体积; (2)求证:A 1 C⊥平面BDE. |
(1)
(2)证明略
(1)∵CE=
CC 1 =
,
∴V C —BDE =V E —BCD =
S △BCD ·CE
=
×
×1×1×
=
.
(2)证明 连接AC、B 1 C.
∵AB=BC,∴BD⊥AC.
∵A 1 A⊥底面ABCD,
∴BD⊥A 1 A.
∵A 1 A∩AC=A,
∴BD⊥平面A 1 AC.
∴BD⊥A 1 C.
∵tan∠BB 1 C=
=
,
tan∠CBE=
=
,∴∠BB 1 C=∠CBE.
∵∠BB 1 C+∠BCB 1 =90°,
∴∠CBE+∠BCB 1 =90°,∴BE⊥B 1 C.
∵BE⊥A 1 B 1 ,A 1 B 1 ∩B 1 C=B 1 ,
∴BE⊥平面A 1 B 1 C,∴BE⊥A 1 C.
∵BD∩BE=B,BE
平面BDE,BD
平面BDE,
∴A 1 C⊥平面BDE.
![](http://img.wesiedu.com/upload/f/92/f92079ec236223a13b1f0ada93ede0ad.jpg)
(1)∵CE=
![](http://img.wesiedu.com/upload/9/80/98025afb61cb47ebda6bf70e67a6e7e0.jpg)
![](http://img.wesiedu.com/upload/c/e2/ce2c8e615d454b0075eeffaf1bc87bd9.jpg)
![](http://img.wesiedu.com/upload/e/b2/eb2d9e78fe39fd00bf0dd9b82c5f6fde.jpg)
∴V C —BDE =V E —BCD =
![](http://img.wesiedu.com/upload/3/1d/31d88e0536a631564cb60a868a302452.jpg)
=
![](http://img.wesiedu.com/upload/3/1d/31d88e0536a631564cb60a868a302452.jpg)
![](http://img.wesiedu.com/upload/c/e2/ce2c8e615d454b0075eeffaf1bc87bd9.jpg)
![](http://img.wesiedu.com/upload/c/e2/ce2c8e615d454b0075eeffaf1bc87bd9.jpg)
![](http://img.wesiedu.com/upload/f/92/f92079ec236223a13b1f0ada93ede0ad.jpg)
(2)证明 连接AC、B 1 C.
∵AB=BC,∴BD⊥AC.
∵A 1 A⊥底面ABCD,
∴BD⊥A 1 A.
∵A 1 A∩AC=A,
∴BD⊥平面A 1 AC.
∴BD⊥A 1 C.
∵tan∠BB 1 C=
![](http://img.wesiedu.com/upload/7/76/776f7948064217adcb26890ce0f82105.jpg)
![](http://img.wesiedu.com/upload/c/e2/ce2c8e615d454b0075eeffaf1bc87bd9.jpg)
tan∠CBE=
![](http://img.wesiedu.com/upload/1/c0/1c0a1cf60d2d18c5500bedeb48eb1b7b.jpg)
![](http://img.wesiedu.com/upload/c/e2/ce2c8e615d454b0075eeffaf1bc87bd9.jpg)
∵∠BB 1 C+∠BCB 1 =90°,
∴∠CBE+∠BCB 1 =90°,∴BE⊥B 1 C.
∵BE⊥A 1 B 1 ,A 1 B 1 ∩B 1 C=B 1 ,
∴BE⊥平面A 1 B 1 C,∴BE⊥A 1 C.
∵BD∩BE=B,BE
![](http://img.wesiedu.com/upload/1/31/131fd1206979f7b32af86ff651d70afa.jpg)
![](http://img.wesiedu.com/upload/1/31/131fd1206979f7b32af86ff651d70afa.jpg)
∴A 1 C⊥平面BDE.
如图所示,在长方体ABCD—A 1 B 1 C 1 D 1 中,AB=BC=1,BB 1 =2,
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