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Bapbuong.binh.52
2 năm trước
đặt một điểm sáng S tại tiêu điểm chính của một thấu kính hội tụ có trục chính Δ rồi liên tiếp dịch chuyển S trên Δ theo ra xa thấu kính .gọi S' là ảnh của S tạo bời thấu kính.khi S cách thấu kính 25cm thì khoảng SS' ngắn nhất.tiêu cự của thấu kính có giá trị gần nhất là A.123 cm B.10 cm C.không tính được D.0 cm giải thích nữa
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