已知cos（a+?π）=3/5,π/2≤a＜3π/2.求cos2a.
更改：已知cos（a+π/4）=3/5
人气：120 ℃ 时间：2020-04-04 17:08:53
解答
因为 π/2 ≤a ＜3π/2,
所以 3π/4 ≤a +π/4 0,
所以 3π/2
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