IV. TIEN TDM DAY HOC
c) Qud trinh thuc hien
• Neu hai tinh chat 1 va 2, hudng đn chumg minh hai tinh chat nay
Vdi hai vecto bat ki a, b va mpi sd thuc k, I, ta cd
1) k(ld) = (kl)d; 2) (k + l)d = kd + ld ;
Ta cd the chirng minh hai tinh chat 1) va 2) nhu sau:
TCI: Ta dt chifng minh hai vecta cd dp dai bang nhaụ
Néu k>0, l>0 hai vecto nay cung chieu vdi ạ Do đ chiing cimg chiaụ Neu k>0,l<0:kl<0^ (kl)a ngupc chiau vdi a, la ngupc chieu vdi
a=^ k(la) ngupc chieu vdi ạ Do đ hai vecta nay ngupc chieu vdi a, nan
chiing cung chieụ Cac trudng hpp cdn lai chiing minh tuang tu ta cd : Hai vecto nay ciing chieu va ciing dp daị
Tinh chát 2) GV tu chiing minh.
• Neu cac tinh chat cdn iai
3)k(d + b) = kd + kb ; k(d-b) = k d-kb ; 4) kd -0 khi vd ehi khi k = 0 hodc 5 = 0.
• Thurc hien TL 2 trang 20 SGK.
GV : Thuc hien thao tac nay trong 5'.
Hoat đng cua GV Cdu hdi 1 ^, , , ^, BA . BC Iinh tl so va . BÁ BC Cdu hdi 2
So sanh chieu ciia hai vecto AC
v a A ' C .
Hoat đng cua HS
Ggi y trd ldi cdu hdi 1 BA BC 1 BÁ BC 3' Ggi y trd ldi edu hdi 2
Cdu hdi 3
Hay ket thiic viec chiing minh tinh chat 3 bdng each dung quy tdc ba diem.
Ggi y trd ldi edu hdi 3 3AB + 3BC = ÁB + BC
= A ' C ' = 3AC.
• GV neu chii y quan trong.
1) Do tinh chát 1, ta cd (-k)d = (-1 .k)d = (-l)(kd) = -(kd). Bdi vay ca hai vecto (-k)d va -(kd) dau cd the viet dcfn gian la -kạ
2) Vecto — 5 cd the viet la . Chang han - 5 ed the viét la—.
n n ' 3 3
• Cho HS giai bai toan 1, nhdm ren luyen ki nang ve tfch ciia vecto vdi mdt sd
- Treo hinh 22 len bang.
- Cho HS phan tich: MA = MI + IA va MB = Ml + IB.
- Gpi mdt HS thuc hien IA + IB - Riit ra két luan bai toan.
Hinh 22
GVco the chung minh bang each khac : dung D sao cho AMBD la hinh binh
hanh. Theo quy tdc hinh binh hanh ta cd:
MA + M 5 = MD = 2M/.
• Thuc hien - ^ 3 trang 20 SGK.
GV : Thirc hien thao tac nay trong 5'. (GV ve hinh 23 trong SGK len bang)
Hoat đng cua GV
Cdu hdi 1
Tuong tu bai toan 1, hay bieu thi
cac vecto MA, MB va MC qua cac vecto MG, GA, GB vaGC.
Cdu hdi 2
Tinh long MA + MB + MC.
Cdu hdi 3
Két luan.
Hoat đng cua HS
Ggi y trd ldi cdu hdi 1 MA = MG + GA MB = MG + GB MC = MG + GC. Ggi y trd ldi cdu hdi 2
MA+ MB + MC
= 3MG + GA + GB + GC. Ggi y trd ldi cdu hdi 3
MA+ MB + MC =3MG.
HOAT DONG 3