Gio n: °»‌ng th³ng vu´ng g³c v»›i m·t ph³ng

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S GD v T Qung Tr Trng THPT Th x Qung Tr ------------ ------------

Giao an bai day: 3: NG THANG VUONG GOC VI MAT PHANG( Tiet 36 ) Gio vin hng dn Sinh vin thc tp Lp thc tp Thi gian28/3/2013

: Trng Th My Dung : Tran Quang : 11A4 : Tiet 7, th 3,

I. Mc tiu. V kin thc: - Nm chc nh ngha ng thng vung gc vi mt phng. - Nm v vn dng c iu kin ng thng vung gc vi mt phng - Nm c nh ngha mt phng trung trc ca on thng. V k nng: - HS bit cch chng minh chng minh ng thng vung gc vi mt phng. Trn c s vn dng chng minh hai ng thng vung gc vi nhau. - S dng mt phng trung trc on thng tm qu tch tp hp im n gin. - V hnh khng gian p, chnh xc. V thi . - Nghim tc, cn thn, tch cc hc tp v xy dng bi. II. Chun b ca GV: + Gio n in t, SGK, SBT v my chiu. III. Phng php dy hc

GV kt hp din gii, hi p vi hot ng nhm. IV. Tin trnh bi hc. HDHS

HDGV Cu hi: Cho t din ABCD sao cho AB vung gc vi BC v BD. Bng phng php vect, cmr AB vung gc vi CD.

A

D B

u u u ru u u r u u u r u u u r u u u r u u u ru u u r u u u ru u u r Ta c: AB.CD = AB.(CB + BD) = AB.CB + AB.BD = 0

C

1. n nh lp. 2. Kim tra bi c.

t vn : Gi s ng thng vung gc r r r vi hai ng thng ct nhau a v b. Gi n , a , b ln lt l vecto ch phng ca , a v b sao cho 3 vecto ny c chung im gc nh hnh v di. Xt ng thng c bt k nm trong mt phng (a,b). V a, b, c ng r phng nn nu c l vecto ch phng ca c th tn ti duy nht hai s thc , sao cho r r r c = a + b .

Khi n.c = n. ( a + b ) = a.n + b.n = 0 . Vy n c hay vung gc vi mir r

rr

r

r

r

rr

rr

ng thng c nm trong mt phng (a,b). Khi ta ni vung gc vi mt phng (a,b). 3. Bi mi. HDGV GV pht biu nh ngha th no l ng thng vung gc vi mt phng. GV pht biu L 1 c chng minh trn v lin h vi bi tp kim tra bi c. Bng L 1, ta c ngay kt lun AB vung gc vi CD v AB vung gc vi mt phng (BCD). Trn c s ta gi quyt c Hi 1. Tnh cht 1: Nh vy, qua im O ta k ln lt hai ng thng b v c phn bit cng vung gc vi d th ta c mt phng (P) to bi b d v c. chng minh tnh cht 1 : Gi s c hai mt phng (P) v (Q) cng i qua O v vung gc vi (d). Gi d l giao tuyn ca (P) v (Q). Mt phng (d,d) ct hai mt phng trn theo cc giao tuyn a v b. Khi a, b v d cng vung gc vi d nn khng ng phng. iu ny v l. Vy Gi thit phn chng l sai. Tnh cht 2: dng c ng thng trong tnh cht HDHS Ni dung trnh chiu nh ngha 1: Mt ng thng d gi l vung gc vi mt phng (P) nu d vung gc vi mi ng thng nm trong mt phng (P). K hiu: d (P). nh l 1: Nu ng thng d vung gc vi hai ng thng ct nhau nm trong (P) th d vung gc vi (P). Tnh cht 1: C duy nht mt phng (P) i qua im O cho trc v vung gc vi ng thng (d) cho trc.

Tnh cht 2: C duy nht ng thng ( ) i qua im O cho trc v vung gc vi mt

2, ta ly hai ng thng a v b ct nhau cng nm trong mt phng (P) ri dng hai mt phng (Q) v (R) cng i qua O v ln lt vung gc vi a v b. Khi giao tuyn ca (Q) v (R) chnh l ng thng cn dng. Chng minh tnh cht 2: Gi s c hai ng thng v cng i qua O v vung gc vi (P). Khi gi A v B ln lt l giao im ca v vi (P). Khi tam gic ABO c 2 gc vung ti A v B. iu ny v l. H: Ti sao ta c MA = MB ?

phng (P) cho trc.

Nhn xt: Cho on thng AB c trung im O. Gi (P) l mt phng qua O v vung gc vi on (ng) thng AB. Theo tnh cht 1 ta c duy nht mt mt phng (P) tha iu kin trn. (P) c gi l mt phng trung trc ca on thng AB. Vi mi im M nm trn mt T : V tam phng (P), ta lun c MA = MB. gic MAB c MO va l ng cao va l trung trc nn cn ti M. Vy MA = MB.

H: Tm tp hp tt c cc im M cch u 3 nh ca tam T : M cch gic ABC? u A v B nn nm trn mt phng trung trc ca AB. M cch u B v C nn M thuc mt phng trung trc ca

GV a ra bi ton. Gii bi ton: a. AB AD AB AS AB ( SAD) AD AS = A

BC. Vy M nm trn giao tuyn ca hai mt phng ny. Bi Ton: Cho hnh chp S.ABCD c y l hnh vung, SA vung gc vi mp(ABCD). a. Chng minh AB vung gc vi mt phng (SAD). b. Chng minh CD vung gc vi mt phng (SAD). c.Chng minh tam gic SBC vung ti B. d. Chng minh BD vung gc vi SC.

b. CD AD AB AS CD ( SAD) AD AS = A

c. BC AB AB AS BC ( SAB ) AB AS = A BC BS BD AC BD AS BD ( SAC ) AC AS = A BD SC

d.

Kt thc bi ton: b, ta bit rng CD song song vi AB. Vy c cch no chng minh CD vung gc vi mt phng (SAD) nhanh gn hn cch trn khng khi m ta bit AB vung gc vi (SAD)?Bi gii s ngn gn hn khi ta c cc tnh cht quan h gia song song v vung gc m ta s hc tit sau c ta c cng c mi chng minh hai ng thng vung gc vi nhau. d, ta ch cn ch ra AC vung gc vi BD l , chnh l ni dung ca nh l ba ng vung gc. 4. Dn d : - c li nh l 1 v 2 tnh cht hc.

- c trc cc mc 3, 4 v 5 trong SGK.

V.

Rt kinh nghim.

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Qung tr, ngy 28 thng 03 nm 2013.

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