题目

已知cos(α+β)=,cos(α-β)=-,＜α+β＜2π,＜α-β＜π,求cos2α. 答案：解:cos2α=cos［(α+β)+(α-β)］=cos(α+β)cos(α-β)-sin(α+β)sin(α-β),∵＜α+β＜2π,∴sin(α+β)=-,＜α-β＜π,∴sin(α-β)=.∴cos2α=×(-)-(-)×=-.It's time that we went home. Let's go and ________ Mr Green good night. [　　] A．wishB．keep C．sayD．hope