cosa=1/3,-π/2＜a＜0
那么sina=-√[1-(1/3)^2]=-2√2/3
tana=sina/cosa=-2√2
所以cos(-a-π)sin(2π+a)/cos(-a)tana
=-cos(-a)sina/cosatana
=-cosasina/sina
=-cosa
=--1/3
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