题目

There was a man playing the piano in a bar. He was a good piano player and always performed in this bar. People came in just to hear him play. But one night, a customer told him that he didn’t want to hear him just play the piano any more. He wanted him to sing a song.The man said, “I don’t sing.”But the customer was persistent(执着的). He told the bar manager, “I’m tired of listening to the piano. I want that man to sing！” The manager shouted across the room, “If you want to get paid, sing a song. Our customers are asking you to sing！”So he did He sang a song. A piano已知直三棱柱ABC-A1B1C1，底面△ABC是等腰三角形，∠BAC=120°，，CN=3AN，点M，P，Q分别是AA1，A1B1，BC的中点．（Ⅰ）求证：直线PQ∥平面BMN；（Ⅱ）求直线AB与平面BMC所成角的正弦值．