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Cho 2 nguon sog $S_1$ va $S_2$ cach nhau $8cm$. ve 1 phja $S_1S_2$ lay them 2 diem $S_3S_4$ sao cho $S_3S_4=4cm$ va hop thah hjh thag can $S_1S_2S_3S_4$. biet $ \lambda =1cm$. hoi duog cao cua hjh thang lon nhat la bn de tren $S_3S_4$ co $5$ diem dd cuc dai
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