Which is the thing which is not assembled definitely? 正しくない展開図は どれ？（開成中学 １９７５年）

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It is the three-dimensional sketch which figure A cuts the tetrahedron which I made with four pieces of equilateral triangles of the same size on a parallel plane in a base, and it was possible for.
Figure B is development of figure A.
In figure C, there are some development of figure A, which is the thing which is not assembled definitely?

図Aは同じ大きさの正三角形4枚で作った三角錐を底面に平行な平面で切ってできた立体の見取り図です。図Bは図Aの展開図です。図Cの中にも、図Aの展開図がいくつかありますが、正しく組み立てられないものはどれでしょうか？

こたえAnswer

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スマートホンアプリ「立方体の切り口はどんな形？」（ネット環境でのＦｌａｓｈアニメーション）

スマホ向け解法集→「中学受験ー算数解き方ポータル」