专题06空间中的平行与垂直
【要点提炼】
1.直线、平面平行的判定及其性质
(1)线面平行的判定定理:a⊄α,b⊂α,ab⇒aα.
(2)线面平行的性质定理:aα,a⊂β,α∩β=b⇒ab.
(3)面面平行的判定定理:a⊂β,b⊂β,a∩b=P,aα,bα⇒αβ.
(4)面面平行的性质定理:αβ,α∩γ=a,β∩γ=b⇒ab.
2.直线、平面垂直的判定及其性质
(1)线面垂直的判定定理:m⊂α,n⊂α,m∩n=P,l⊥m,l⊥n⇒l⊥α.
(2)线面垂直的性质定理:a⊥α,b⊥α⇒ab.
(3)面面垂直的判定定理:a⊂β,a⊥α⇒α⊥β.
(4)面面垂直的性质定理:α⊥β,α∩β=l,a⊂α,a⊥l⇒a⊥β.
考点
考向一空间点、线、面位置关系
【典例1】(1)(2020·河南百校大联考)如图,正方体的底面与正四面体的底面在同一平面α上,且ABCD,若正方体的六个面所在的平面与直线CE,EF相交的平面个数分别记为m,n,则下列结论正确的是( )
A.m=nB.m=n+2
C.m<nD.m+n<8
(2)(2019·北京卷)已知l,m是平面α外的两条不同直线.给出下列三个论断:
①l⊥m;②mα;③l⊥α.
以其中的两个论断作为条件,余下的一个论断作为结论,写出一个正确的命题:________.
解析(1)直线CE⊂平面ABPQ,从而CE平面A1B1P1Q1,
易知CE与正方体的其余四个面所在平面均相交,
则m=4.
取CD的中点G,连接FG,EG.
易证CD⊥平面EGF,
又AB⊥平面BPP1B1,AB⊥平面AQQ1A1且ABCD,
从而平面EGF平面BPP1B1平面AQQ1A1,
∴EF平面BPP1B1,EF平面AQQ1A1,
则EF与正方体其余四个面所在平面均相交,n=4,
故m=n=4.
(2)已知l,m是平面α外的两条不同直线,由①l⊥m与②mα,不能推出③l⊥α,因为l可能与α平行,或l与α相交但不垂直;
由①l⊥m与③l⊥α能推出②mα;
由②mα与③l⊥α可以推出①l⊥m.
故正确的命题是②③⇒①或①③⇒②.
答案(1)A(2)若mα,l⊥α,则l⊥m(或若l⊥m,l⊥α,则mα,答案不唯一)
探究提高1.判断空间位置关系命题的真假
(1)借助空间线面平行、面面平行、线面垂直、面面垂直的判定定理和性质定理进行判断.
(2)借助空间几何模型,如从长方体、四面体等模型中观察线面位置关系,结合有关定理,进行肯定或否定.
2.两点注意:(1)平面几何中的结论不能完全引用到立体几何中;(2)当从正面入手较难时,可利用反证法,推出与题设或公认的结论相矛盾的命题,进而作出判断.
【拓展练习1】
(2020·衡水中学调研)
A.过点M有且只有一条直线与直线AB,B1C1都相交 |
B.过点M有且只有一条直线与直线AB,B1C1都垂直 |
C.过点M有且只有一个平面与直线AB,B1C1都相交 |
D.过点M有且只有一个平面与直线AB,B1C1都平行 |
【知识点】 平面的基本性质及辨析
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A.![]() | B.![]() ![]() |
C.![]() | D.平面![]() ![]() |
【知识点】 证明异面直线垂直 线面关系有关命题的判断 证明面面垂直
考向二空间平行、垂直关系的证明
【典例2】(2019·北京卷)如图,在四棱锥P-ABCD中,PA⊥平面ABCD,底面ABCD为菱形,E为CD的中点.
(1)求证:BD⊥平面PAC;
(2)若∠ABC=60°,求证:平面PAB⊥平面PAE;
(3)棱PB上是否存在点F,使得CF平面PAE?说明理由.
(1)证明因为PA⊥平面ABCD,BD⊂平面ABCD,
所以PA⊥BD.
因为底面ABCD为菱形,
所以BD⊥AC.
又PA∩AC=A,
所以BD⊥平面PAC.
(2)证明因为PA⊥平面ABCD,AE⊂平面ABCD,
所以PA⊥AE.
因为底面ABCD为菱形,∠ABC=60°,且E为CD的中点,
所以AE⊥CD.又因为ABCD,所以AB⊥AE.
又AB∩PA=A,所以AE⊥平面PAB.
因为AE⊂平面PAE,所以平面PAB⊥平面PAE.
(3)解棱PB上存在点F,使得CF平面PAE.理由如下:
取PB的中点F,PA的中点G,连接CF,FG,EG,
则FGAB,且FG=AB.
因为底面ABCD为菱形,且E为CD的中点,
所以CEAB,且CE=AB.
所以FGCE,且FG=CE.
所以四边形CEGF为平行四边形.所以CFEG.
因为CF⊄平面PAE,EG⊂平面PAE,
所以CF平面PAE.
探究提高1.利用综合法证明平行与垂直,关键是根据平行与垂直的判定定理及性质定理来确定有关的线与面,如果所给的图形中不存在这样的线与面,要充分利用几何性质和条件连接或添加相关的线与面.
2.垂直、平行关系的证明,主要是运用转化与化归思想,完成线与线、线与面、面与面垂直与平行的转化.在论证过程中,不要忽视定理成立的条件,推理要严谨.
【拓展练习2】
(2020·石家庄调研)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/29/05af1bc9-0be8-43ab-839e-5dca4d4ade4c.png?resizew=143)
(1)求证:BF∥平面ADP;
(2)已知O是BD的中点,求证:BD⊥平面AOF.
考向三平面图形中的折叠问题
【典例3】图①是由矩形ADEB,Rt△ABC和菱形BFGC组成的一个平面图形,其中AB=1,BE=BF=2,∠FBC=60°.将其沿AB,BC折起使得BE与BF重合,连接DG,如图②.
(1)证明:图②中的A,C,G,D四点共面,且平面ABC⊥平面BCGE;
(2)求图②中的四边形ACGD的面积.
(1)证明由已知得ADBE,CGBE,所以ADCG,
所以AD,CG确定一个平面,从而A,C,G,D四点共面.
由已知得AB⊥BE,AB⊥BC,且BE∩BC=B,BE,BC⊂平面BCGE,
所以AB⊥平面BCGE.
又因为AB⊂平面ABC,所以平面ABC⊥平面BCGE.
(2)解如图,取CG的中点M,连接EM,DM.
因为ABDE,AB⊥平面BCGE,所以DE⊥平面BCGE,又CG、EM⊂平面BCGE,故DE⊥CG,DE⊥EM.
由已知,四边形BCGE是菱形,且∠EBC=60°,得EM⊥CG,
又DE∩EM=E,DE,EM⊂平面DEM,故CG⊥平面DEM.
又DM⊂平面DEM,因此DM⊥CG.
在Rt△DEM中,DE=1,EM=,
故DM=2.又CG=BF=2,
所以四边形ACGD的面积为S=2×2=4.
探究提高1.解决与折叠有关问题的关键是找出折叠前后的变化量和不变量,一般情况下,折线同一侧的线段的长度是不变量,而位置关系往往会发生变化,抓住不变量是解决问题的突破口.
2.在解决问题时,要综合考虑折叠前后的图形,既要分析折叠后的图形,也要分析折叠前的图形,善于将折叠后的量放在原平面图形中进行分析求解.
【拓展练习3】
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(Ⅱ)当平面
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【知识点】 根据体积计算几何体的量 证明线面垂直 面面垂直证线面垂直
考向四空间线面关系的开放性问题
【典例4】(2020·九师联盟检测)如图,四棱锥P-ABCD中,底面ABCD是边长为2的菱形,∠BAD=,△PAD是等边三角形,F为AD的中点,PD⊥BF.
(1)求证:AD⊥PB;
(2)若E在线段BC上,且EC=BC,能否在棱PC上找到一点G,使平面DEG⊥平面ABCD?若存在,求出三棱锥D-CEG的体积;若不存在,请说明理由.
(1)证明∵△PAD是等边三角形,F是AD的中点,∴PF⊥AD.
∵底面ABCD是菱形,∠BAD=,∴BF⊥AD.
又PF∩BF=F,∴AD⊥平面BFP.
由于PB⊂平面BFP,∴AD⊥PB.
(2)解能在棱PC上找到一点G,使平面DEG⊥平面ABCD.
由(1)知AD⊥BF,∵PD⊥BF,AD∩PD=D,
∴BF⊥平面PAD.
又BF⊂平面ABCD,∴平面ABCD⊥平面PAD,
又平面ABCD∩平面PAD=AD,且PF⊥AD,PF⊂平面PAD,∴PF⊥平面ABCD.
连接CF交DE于点H,过H作HGPF交PC于G,
∴GH⊥平面ABCD.
又GH⊂平面DEG,∴平面DEG⊥平面ABCD.
∵ADBC,∴△DFH∽△ECH,
∴=
=
,∴
=
=
,
∴GH=PF=
,
∴VD-CEG=VG-CDE=S△CDE·GH
=×
DC·CE·sin
·GH=
.
探究提高1.求解探究性问题常假设题中的数学对象存在(或结论成立),然后在这个前提下进行逻辑推理,若能推导出与条件吻合的数据或事实,说明假设成立,并可进一步证明;若推导出与条件或实际情况相矛盾的结论,则说明假设不成立.
2.解决空间线面关系的探究性问题,应从平面图形中的平行或垂直关系入手,把所探究的结论转化为平面图形中线线关系,从而确定探究的结果.
【拓展练习4】
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(1)在棱PB上是否存在点E,使得AE
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/895d6f710d5f67e1d4c7408d50d77281.png)
(2)若△PBC的面积为
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0181723c3b13b5ab2f33d9cdcdd8993e.png)
【专题拓展练习】
一、单选题
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A.![]() | B.![]() | C.![]() | D.![]() |
【知识点】 锥体体积的有关计算 由二面角大小求线段长度或距离
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A.![]() ![]() |
B.存在平面![]() ![]() ![]() |
C.存在平面![]() ![]() ![]() ![]() |
D.存在直线![]() ![]() ![]() |
【知识点】 线面关系有关命题的判断 面面关系有关命题的判断
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A.![]() | B.![]() | C.![]() | D.![]() |
【知识点】 求异面直线所成的角
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A.若![]() ![]() | B.若![]() ![]() |
C.若![]() ![]() | D.若![]() ![]() |
【知识点】 线面关系有关命题的判断 面面关系有关命题的判断
A.三点确定一个平面 |
B.垂直于同一直线的两条直线平行 |
C.若直线![]() ![]() ![]() |
D.若![]() ![]() ![]() ![]() |
【知识点】 平面的基本性质及辨析
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A.![]() | B.![]() | C.![]() | D.![]() |
【知识点】 求异面直线所成的角
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A.![]() ![]() | B.![]() ![]() |
C.![]() ![]() | D.![]() ![]() |
【知识点】 判断线面平行
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A.![]() | B.![]() | C.![]() | D.![]() |
【知识点】 求异面直线所成的角
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A.若![]() ![]() ![]() ![]() |
B.若![]() ![]() ![]() ![]() |
C.若![]() ![]() ![]() |
D.若![]() ![]() ![]() ![]() |
【知识点】 线面关系有关命题的判断 面面关系有关命题的判断
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0f85fca60a11e1af2bf50138d0e3fe62.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/294f5ba74cdf695fc9a8a8e52f421328.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b6a24198bd04c29321ae5dc5a28fe421.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e170f206fdbbd834aad7580c727e2cc6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b5858ee1ce52b251816757257a11c29.png)
①若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/21d73ad9021fc4df50106faf32845d2d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/96408d949b99a284bf850aa7d9ac41cb.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7a93a52c2b943e4c70ace99ed802d2b5.png)
②若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0fe920cd78db25f5b4df37d066e57800.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/808c6d37467a5c995d71e49408503927.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a5986f2991d45fbf3578f08f27d9fd7e.png)
③若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4042f9c51f83e3367d496e851735d7f9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/79018590293277ff2d76452a50ad2dbc.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7a93a52c2b943e4c70ace99ed802d2b5.png)
④若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a1b610e3c5b3d78a5730e7f3d736ac28.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a5986f2991d45fbf3578f08f27d9fd7e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4042f9c51f83e3367d496e851735d7f9.png)
其中假命题是
A.① | B.② | C.③ | D.③④ |
【知识点】 线面关系有关命题的判断 面面关系有关命题的判断
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e170f206fdbbd834aad7580c727e2cc6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b5858ee1ce52b251816757257a11c29.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9e380108ba2cf04e68a5a9393d2b921c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f157205cb5cb4a538b09d989f2d9ae95.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/384a65ca1ee3d7d86b988ca34c885e18.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1dedfa42c16dd0aefa2928a6e41f3dba.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/18839099fb51cb1dda11653615ad0a5c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a5986f2991d45fbf3578f08f27d9fd7e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a5986f2991d45fbf3578f08f27d9fd7e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/18839099fb51cb1dda11653615ad0a5c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1dedfa42c16dd0aefa2928a6e41f3dba.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/384a65ca1ee3d7d86b988ca34c885e18.png)
A.1 | B.2 | C.3 | D.4 |
【知识点】 线面关系有关命题的判断 面面关系有关命题的判断
二、解答题
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fc11331a7b2d2619b40ee6d34c3bd620.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/080ca48cd27d4bf9d9ef084b558fc17a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ccd4fd4b7a4d6b8ca0c5827c055a9ce7.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0b80ee363635d73f601654339028daec.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2a30f3a8b673cc28bd90c50cf1a35281.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0dc5c9827dfd0be5a9c85962d6ccbfb1.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/4/177c488e-2ff6-4b10-b832-cd76d10f2df9.png?resizew=165)
(1)求证:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d03c2639a3b3f1f9590080b38ab21374.png)
(2)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ac047e91852b91af639feec23a9598b2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/48f3c9abbd78e9a6840ee5f30381daac.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f4b5e0b8c35a7d9b3d68db8e5c89b8bd.png)
【知识点】 锥体体积的有关计算 线面垂直证明线线垂直
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/42d3a82b8e587ee890467835bc4e854c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2777840758e70e7dbbc18cef8f3d6d2b.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/7/bc0b7dcd-31ec-4425-b82f-6dca32965018.png?resizew=148)
(1)在图中作出平面
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7bef5239ddbb0972700ce01daf9ee7cf.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fed2f706801662432b68797e72647c6e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a9df740160690029ac1e730c85f20347.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ee8b5a6dbcf05f572f83f51abf7d668c.png)
(2)求点C到平面
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fed2f706801662432b68797e72647c6e.png)
【知识点】 平面的基本性质及辨析 证明线面垂直 求点面距离