根据短文内容，选择正确答案。

    A man lived in a village. He had an old cat at home. The cat couldn't run fast and bite，because she was so old. One day, when she saw a mouse, she jumped and caught it. But she could not bite it. The mouse got out of her mouth and ran away.

    Then the man became very angry and began to beat the cat. The cat said, “Don't beat your old servant. I know I'm old and can't kill a mouse. But I have worked for you for many years, and I still want to work for you. Be kind to the old, and remember what good work they did when they were young."

    When the man heard that, he realized that he was wrong.

如图，在四棱锥P﹣ABCD中，底面ABCD为平行四边形，∠ADC=45°，AD=AC=1，O为AC的中点，PO⊥平面ABCD，PO=2，M为PD的中点．

（1）证明：PB∥平面ACM；

（2）求直线AM与平面ABCD所成角的正切值．