1.已知sinα=2/3 则cos（π-2α）=
问题描述：
1.已知sinα=2/3 则cos（π-2α）=
2.已知△ABC中 cotA=-12/5 则cosA=
3.已知secα=13/12 α是第四象限角 则sinα=
求数学开窍...
答
1.cos(π-2α)=-cos2α=2*(sinα)^2-1=2*4/9-1=-(1/9)2.cotA=cosA/sinA=-12/5(1-(sinA)^2)/(sinA)^2=144/25sinA=5/133.secα=1/cosα=13/121/(1-sin^2(α))=169/144sinα=-5/13