已知a²﹣b²=6，a﹣b=2，则a+b=____．

阅读短文，判断正误。
Dear Joe,
    How are you? My sister and I are having our holiday. Yesterday we went to the library. We got there by bus. We read storybooks and had lunch there. The food was delicious. It was cheap, too. We went shopping in the afternoon. I bought a pair of shoes. I was very tired. We are planning to learn skating and do many other things during the holiday. We are sure we'll have a good time.
    How about your holiday? Please write to me soon.
Miss you.
Alice

千位上是6，十位上是2，这个数是____．

I certainly appreciate _____ us about the delay in delivering the materials because we had planned to begin work tomorrow.

如图，在多面体ABCDM中，△BCD是等边三角形，△CMD是等腰直角三角形，∠CMD=90°，平面CMD⊥平面BCD，AB⊥平面BCD．

（Ⅰ）求证：CD⊥AM；

（Ⅱ）若AM=BC=2，求直线AM与平面BDM所成角的正弦值．