题目

4.已知sina=,则的值为( )（A）－                   （B）－                          （C）                      （D ） 答案：答案：A解析：sin4α－cos4α=(sin2α+cos2α)(sin2α－cos2α)=sin2α－cos2α=2sin2α－1=2×()2－1=－. Tim—家和他的朋友们在海滩上玩，他们都在做什么呢？看图写一写吧fly kites paint a picture read a book eat sandwiches watch birds collect shells swim in the seaToday is Sunday. Tim and his friends are playing at the beach. Look, Tim and John are flying kites. Betty……