IV. TIEN TDINH DAY HOC
c) Qud trinh thuc Men • Neu hai quy tac:
• Neu hai quy tac:
GV dat van de cho hoc sinh neu hai quy tac do, c6 suf dung hai hinh 12 va 13 SGK.
Vdi ba diem bát ki M, Â, P, ta cd
MN+ NP = MP.
Neu OABC la hinh binh hanh thi ta cd
^ + 0C = 0B.
• Thuc hien \?2\ trang 12 SGK.
GV thirc hien thao tac nay trong 4'.
Hinh 12
Hinh 13
Hoat đng ciia GV
Cdu hdi 1
Hay chimg td OA + OC = 0 5 .
Cdu hdi 2
Hay giai thich tai sao ta cd
|5 + 5| <|5i + ^ i .
Hoat đng cua HS
Goi V trd ldi cdu hdi 1
Tac60A + 0C = 0A + AB = 0B. Ggi y trd ldi cdu hdi 2
Data = OA, ^ = 0 5 , khi đ ta cd : \a\ + \b\=OA + AB;
\a + b\ =OfịTrongbadiemO, A, 5
ta ed OA + A5 < OB.
• Cho HS lam bai toan 1 trang 12 SGK.
GV cho HS ve tCr giac loi ABDC de de thao tac qua trinh giai bai toan.
- Diing quy tac tam giac tacd AC = AD + DC.
- Dung tinh chat giao hoan.
AC + BD = AD + DC + BD = AD + BD + DC=^AD + BC.
GV tong quat cho 4 diem A, B, C, D bat kị • Thirc hien & 5 trang 12 SGK.
GV thiTc hien thao tac nay trong 3'.
Hoat dong ciia GV
Cdu hdi 1
Hay chimg td ^ ' = Z s + 5C
Cdu hdi 2
Hay chung minh
Jc + 'BD ^JD + ~BC.
Hoat dong cua HS
Ggi y trd ldi cdu hdi 1
Theo quy tic tam giac.
Ggi y trd ldi cdu hdi 2
Tac6AC = AB + BC nen AC+BD=AB+BC+BD
• Cho HS lam bai toan 2 trang 12 SGK.
Sii-dunghinh 14SGK.
- Xac dinh vecto tdng: AB + AC = AD .
- Tinh AD = 2 AH.
GV CO the dSt them cac cau hoi
- Tinh dp dai cua cac vecto CA + ^;BA + W.
- Em cd nhan xet gi vd dp dai cua cac vecto tren vdi chieu cao ciia tam giac
deu ABC?
Cho hoc sinh tra ldi va GV nhan xet.
• Cho HS lam bai toan 3 trang 13 SGK. ^
-Sit dung hinh 15 SGK.
Tinh MA + AM = MM = 0.
- Sir dung AM = MB.
-Tinh GA + GB = GC' = CG TinhGA + GB + GC.
• Thuc hien [?3j trang 13 SGK.
GV thirc hien thao tac nay trong 3'.
Hinh 15
Hoat dong cua GV
Cdu hdi 1
Em cd nhan xet gi ve dp dai cua CG
vaGM. Cdu hdi 2
Em ed nhan xet gi ve dp dai cua
GC'vaGM.
Hoat dong cua HS
Ggi y trd ldi cdu hdi 1 CG = 2GM
Ggi y trd ldi cdu hdi 2 GC' = 2GM
Cdu hdi 3
Em cd nhan xet gi ve dp dai cua CG
vaGC'.
Cdu hdi 4
Em ed nhan xet gi vd hudng cua hai
vecto CG vaGC'.
Ggi y trd ldi cdu hdi 3
Hai doan thang nay bang nhaụ
Ggi y trd ldi cdu hdi 4
Hai vecta nay ciing hudng.
• Ghi nhd
Niu M Id trung diim dogn thdng AB thi MA + MB = 0 ;
• Chuy
Níu G Id trgng tdm tam gide ABC thi GA + GB + GC = 0.
Quy tdc hinh binh hdnh thudng duge dp dung trong vdt li de xdc dinh hgp lite cua hai lite cung tdc dgng lin mgt vdt.
TOM TST BAI HOC
1. Cho hai vecto a vab. Lay mdt diem A nao đ rdi xac dinh cac diem 5 va C sao
cho AB = a, BC = b. Khi đ vecta AC dupe gpi la tdng cua hai vecta 5 va b. Ki
hi6u
JC= a + b.
Phep láy tdng ciia hai vecto dupe gpi la phep cdng vectọ 2. Tinh chat
I) Tinh chat giao hoan : a + b =b + d;
1) Tinh chat ket hpp : (d + b) + c =d + (b + c);
3. Quy tac
Quy tac ba diem :
Vdi ba diem bat ki M, N, P, ta cd: WN + NP = MP.
Quy tac hinh binh hanh :
Neu OABC la hinh binh hanh thi ta cd: OA + OC = OB.
4. Neu M la trung diem doan thang AB thi MA + MB = 0;
Ndu G la trpng tam tam giac ABC thi GA + GB + GC = 0.
HOAT DONG 4