sina=cos2α,α属于(π/2,π),则cotα的值为?

sina=cos2α,α属于(π/2,π),则cotα的值为?
sina=cos2α,α属于(π/2,π),则cotα的值为?

sina=cos2α,α属于(π/2,π),则cotα的值为?
sina=cos2α=1-2(sina)^2
设sina=t
t=1-2t^2
2t^2+t-1=0
(2t-1)(t+1)=0
t=1/2 or -1
因为α属于(π/2,π)
所以sina=1/2 cosa