>
数学
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如图,在正方体ABCD-A
1
B
1
C
1
D
1
中,点N在BD上,点M在B
1
C上,且CM=DN,求证:MN∥平面AA
1
B
1
B.
人气:120 ℃ 时间:2019-08-21 17:39:08
解答
解
:如图,作MP∥BB
1
,交BC于点P,连接NP.
∵MP∥BB
1
,∴
CM
M
B
1
=
CP
PB
.
∵BD=B
1
C,DN=CM,∴B
1
M=BN.
∵
CM
M
B
1
=
DN
NB
,∴
CP
PB
=
DN
NB
.
∴NP∥CD∥AB.∴面MNP∥面AA
1
B
1
B.
∴MN∥面AA
1
B
1
B.
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