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Hai gương phẳng G1, G2 có mặt phản xạ quay vào nhau và hợp với nhau một góc α. Tia sáng SI đến G1, phản xạ theo phương IJ đến G2 rồi phản xạ tiếp theo phương JR. a) Với α là góc nhọn. Tìm góc hợp bởi tia SI và tia JR. b) Khi α là góc vuông. Chứng minh SI // JR.
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