Vật sáng AB cao 3 cm đặt vuông góc với trục chính của một thấu kính hội tụ có f = 12cm. Vật AB cách thấu kính một khoảng bằng 30cm. a) Tính: khoảng cách từ ảnh đến thấu kính và chiều cao của ảnh A'B'. b) Di chuyển vật AB dọc theo trục chính đến gần thấu kính để có được ảnh ảo cao gấp 3 lần vật. Hãy tính khoảng cách từ vật đến thấu kính.

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