1. Một tam giác có số đo diện tích (cm²) bằng số đo chu vi (cm) . Tính bán kính r của đường tròn nội tiếp tam giác đó A. r =2 B. r=4 C. r =3 D. r=5 2. Cho đường trong (O ; 6cm) và điểm O' với OO'= 8cm. Tìm giá trị của R để đường tròn (O' ; R) tiếp xúc với dường tròn (O ; R) A. 2 cm B. 2cm hoặc 14cm C. 14cm D. 6cm 3. Cho đường tròn (O ; 10cm) dây AB = 16cm. Khi đó khoảng cách từ tâm O đến dây AB bằn A. 2cm B. 4cm C. 6cm D. 8cm 4. Cho tam giác đều ABC ngoại tiếp đường tròn bán kính 2cm. Tính diện tích S của tam giác ABC

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