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△ABCと△BDCで
共通な角なので∠C=∠C･･･①
△ABCは仮定より二等辺三角形なので
∠ABC=∠C･･･②
△BDCは仮定より二等辺三角形なので
∠BDC=∠C･･･③
②,③より∠ABC=∠BDC･･･④
①,④より、2組の角がそれぞれ等しいから
△ABC∽△BDC
10:7=7:X
X=4.9
答え:4.9cm
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