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簡単に求めるなら、
x＝積/和＝(3×5)/(3+5)＝15/8
相似を使ってちゃんと求めるなら、
△ABE∽△DCEより、BE：EC＝AB：CD
→ BE：EC＝3：5
△BEF∽△BCDより、BE：BC＝FE：DC
→ 3：8＝x：5
→ x＝15/8
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