sina=cos2α,α属于(π/2,π),则cotα的值为?
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![sina=cos2α,α属于(π/2,π),则cotα的值为?](/uploads/image/z/6722410-58-0.jpg?t=sina%3Dcos2%CE%B1%2C%CE%B1%E5%B1%9E%E4%BA%8E%28%CF%80%2F2%2C%CF%80%29%2C%E5%88%99cot%CE%B1%E7%9A%84%E5%80%BC%E4%B8%BA%3F)
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