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anh ấy ko thắng được bởi vì 40<78 nên anh ấy ko thắng được
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Kết quả cuối cùng của phép tính trên là 5.
Giả thiết
Cho tứ giác cánh diều ABCDABCD với:
- AB=ADAB = AD
- CB=CDCB = CD
Điều này đúng với định nghĩa hình cánh diều: có hai cặp cạnh kề bằng nhau.
🧠 Câu a) Chứng minh AC là đường trung trực của BD
🔷 Phân tích
Gọi OO là giao điểm hai đường chéo ACAC và BDBD. Vì AB=ADAB = AD và CB=CDCB = CD, ta có:
- Tam giác ABDABD cân tại AA
- Tam giác CBDCBD cân tại CC
→ Trong tứ giác cánh diều, hai đường chéo vuông góc và đường nối hai đỉnh có cặp cạnh bằng nhau (ở đây là ACAC) sẽ là trục đối xứng.
🧩 Chứng minh
- ACAC cắt BDBD tại OO
- Hai tam giác cân: ABDABD cân tại AA, CBDCBD cân tại CC
- Suy ra: ACAC là đường trung trực của đoạn BDBD
⏬ Vì nó chia BDBD thành hai phần bằng nhau và vuông góc tại điểm OO
✅ Kết luận: AC là đường trung trực của BD
📐 Câu b) Tính số đo góc BB và góc DD, biết:
- Góc A=100∘A = 100^\circ
- Góc C=70∘C = 70^\circ
🧮 Sử dụng tổng góc tứ giác
Trong mọi tứ giác:
∠A+∠B+∠C+∠D=360∘\angle A + \angle B + \angle C + \angle D = 360^\circThay số:
100∘+∠B+70∘+∠D=360∘⇒∠B+∠D=190∘100^\circ + \angle B + 70^\circ + \angle D = 360^\circ \Rightarrow \angle B + \angle D = 190^\circ📏 Tính thêm
Vì hình cánh diều có trục đối xứng là đường chéo ACAC, hai góc ở hai đầu đường chéo BDBD là góc BB và góc DD, nên:
- ∠B=∠D\angle B = \angle D
Khi đó:
∠B+∠D=2∠B=190∘⇒∠B=∠D=95∘\angle B + \angle D = 2\angle B = 190^\circ \Rightarrow \angle B = \angle D = 95^\circ✅ Kết quả:
- Góc B=95∘B = 95^\circ
- Góc D=95∘D = 95^\circ