Câu 4: Cho vật AB vuông góc với trục chính của TKHT, điểm A cách thấu kính 20 cm, cho ảnh ảo lớn hơn vật 3 lần. A. Bằng cách vẽ hãy xác định quang tâm O và các tiêu điểm F, F’. B. Tính tiêu cự của thấu kính

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