vật sáng ab được đặt vuông góc với trục chính thấu kính hội tụ có tiêu cự 18 cm . vật đặt cách thấu kính 12 cm . a) vẽ ảnh của vật ab nêu tính chaast của ảnh . b) tính khOẢNG cách từ ảnh đến thấu kính và chiều cao của ảnh biết vật cao 2cm ai giúp em làm với

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