Chương III. §7. Trường hợp đồng dạng thứ ba

Nhiệt liệt chào mừng các thầy giáo,
cô giáo về dự giờ thăm lớp
1. Nếu hai tam giác bằng nhau thì chúng đồng dạng với nhau.
2. Hai tam giác đồng dạng với nhau thì bằng nhau.
3. Nếu A’B’C’ =  AMN và  AMN ABC
thì  A’B’C’  ABC
S
S
4. Nếu hai cạnh của tam giác này tỉ lệ với hai cạnh của tam giác kia thì hai tam giác đó đồng dạng
Bài 2: (4 điểm) Các mệnh đề sau đây đúng hay sai?
Bài 1: (6 điểm)
Vì MN // BC nên ……. …….
Vì ……. ……. các cặp góc bằng nhau là: ….. là góc chung
Cho tam giác ABC, có MN // BC (hình vẽ).
Điền vào chỗ trống sao cho thích hợp
S
S
AMN
ABC
AMN
ABC
Đ
S
Đ
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HẾT GIỜ
Bằng cách nào nhận biết được ABC và DEF có đồng dạng với nhau hay không ?
Hai tam giác đồng dạng với nhau thì các góc tương ứng bằng nhau. Vậy hai tam giác có các cặp góc tương ứng bằng nhau thì chúng có đồng dạng với nhau không?
Bài toán:
Cho hai tam giác ABC và A’B’C’ với góc A bằng góc A’, góc B bằng góc B’. Chứng minh tam giác ABC đồng dạng với tam giác A’B’C’.
Định lí
Nếu hai góc của tam giác này lần lượt bằng hai góc của tam giác kia thì hai tam giác đó đồng dạng.
Bằng cách nào nhận biết được ABC và DEF có đồng dạng với nhau hay không ?
Hai tam giác đồng dạng với nhau thì các góc tương ứng bằng nhau. Vậy hai tam giác có các cặp góc tương ứng bằng nhau thì chúng có đồng dạng với nhau không?
Bạn Hoa nói đúng hay sai? Vì sao
Bài tập 1:
và
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HẾT GIỜ
Thảo luận nhóm
Nội dung:








Trong các tam giác dưới đây, những cặp tam giác nào đồng dạng với nhau? Hãy giải thích.
A
B
C
D
E
F
M
N
P
A’
B’
C’
D’
E’
F’
M’
N’
P’
Thảo luận nhóm








Trong các tam giác dưới đây, những cặp tam giác nào đồng dạng với nhau? Hãy giải thích.
A
B
C
D
E
F
M
N
P
A’
B’
C’
D’
E’
F’
M’
N’
P’
a/Trong hình vẽ này có bao nhiêu tam giác ?
Có cặp tam giác nào đồng dạng với nhau không?
b/ Hãy tính các độ dài x và y.
c/ Cho biết thêm BD là tia phân giác của góc B. Hãy tính độ dài các đoạn thẳng BC và BD.
?2 .Cho hình vẽ
Mỗi khẳng định sau đúng hay sai?
1. Nếu hai góc của tam giác này bằng hai góc của tam giác kia thì hai tam giác đó bằng nhau.
2. Hai tam giác đều luôn đồng dạng với nhau
3. Hai tam cân có cặp góc ở đỉnh bằng nhau thì đồng dạng với nhau
4.Nếu hai tam giác ABC và DEF có
thì  ABC  DEF
S
S
Đ
Đ
S
Bài tập 2:
Chọn đáp án đúng
Cho ?ABC và OMN cĩ
a)  ABC MNO
S
thì
b)  ABC NOM
c)  ABC OMN
S
S
S
d)  ABC NMO
Bài tập 3
HƯỚNG DẪN VỀ NHÀ
Học thuộc và nắm vững nội dung v� h? th?ng l?i ba trường hợp đồng dạng của hai tam giác b?ng so d? tu duy . So sánh với ba trường hợp bằng nhau của hai tam giác.
Về nhà làm các bài tập 35; 36, 37, 38 ( SGK)
- Xem tru?c c�c b�i t?p trong ph?n
luy?n t?p d? gi? sau ch�ng ta luy?n t?p.
Bài 35 : Chứng minh rằng nếu tam giác A’B’C’ đồng dạng với tam giác ABC theo tỉ số k thì tỉ số của hai đường phân giác tương ứng của chúng cũng bằng k.
Đo chiều cao của bất kì vật nào
Đo khoảng cách giữa hai địa điểm trong đó có một địa điểm không thể tới được
A
B
C
KÍNH CHÚC QUÝ THẦY CÔ GIÁO MẠNH KHỎE
CHÚC CÁC EM HỌC GIỎI.