已知cosa=-4/5,且a∈（∏/2,∏),求sina、cos2a的值
人气：494 ℃ 时间：2020-05-22 06:39:23
解答
a∈（∏/2,∏),所以sina>0
sina=√(1-16/25)=3/5
cos2a
=cosa^2-sina^2
=16/25-9/25
=7/25
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