设cosα=-√5/5 π
问题描述：
设cosα=-√5/5 π
答
cosα=-√5/5
π＜α＜3π/2
∴sinα=-2√5/5
tanα=2
tan(α-β)=(tanα-tanβ)/(1+tanαtanβ)
=[2-(1/3)]/[1+2*(1/3)]
=(5/3)/(5/3)
=1
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