如图, 在三棱柱 ABC﹣ A 1 B 1 C 1 中, 点 B 1 在底面 ABC 内的射影恰好是点 C, 点D 是 AC 的中点, 且 DA=DB.(1)

如图, 在三棱柱 ABC﹣ A 1 B 1 C 1 中, 点 B 1 在底面 ABC 内的射影恰好是点 C, 点D 是 AC 的中点, 且 DA=DB.(1) 证明: AB⊥CC 1 .

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You will feel cold _______ you dont wear a coat.

You will feel cold _______ you dont wear a coat.

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We will make fewer mistakes _______ we are careful enough.

We will make fewer mistakes _______ we are careful enough.

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President Xi encourages all Chinese people to plant trees______ our country can

President Xi encourages all Chinese people to plant trees______ our country can be greener and more

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