（Ⅰ）证明：AC=AB₁；

（Ⅱ）若AC⊥AB₁，∠CBB₁=60°，AB=BC，求二面角A﹣A₁B₁﹣C₁的余弦值．

读图，回答下列各题。

    Polly Cooper is a mom of two girls named Debbie and Milly. Here is what she says about them.

    "Debbie and Milly are so 1 . Debbie is a shy girl. She enjoys doing things by 2 and doesn't like to talk and play with others. Also, she is more selfish (自私的) than Milly. It is 3 to ask her to share her things with others. But Debbie has her good points, too. Schoolwork is easier  4 Debbie, so her exam results are better. She is also good at 5 and wants to be a dancer in the. future (未来).

    "Milly has bigger problems with her schoolwork. She doesn't like studying and doesn't 6 enough time on it, so her exam results are usually worse. However, Milly is much more outgoing than Debbie. Maybe 7 Milly is two years older than Debbie, she is a little more mature(成熟的).

    "I worry about Milly because she isn't sure about her future. She doesn't know what she 8 to do when she leaves school. But I'm 9 worried about Debbie because she isn't as happy as Milly. I don't think parents ever stop worrying about their kids and I'll 10 them both when they leave home."