3.设sin a=3/5,a为第II象限角,则有()A.cos a=4/5 B.cos a=-4/5 C.cos a=±4/5 D.cos a=2/5
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![3.设sin a=3/5,a为第II象限角,则有()A.cos a=4/5 B.cos a=-4/5 C.cos a=±4/5 D.cos a=2/5](/uploads/image/z/14271340-4-0.jpg?t=3.%E8%AE%BEsin+a%EF%BC%9D3%2F5%2Ca%E4%B8%BA%E7%AC%ACII%E8%B1%A1%E9%99%90%E8%A7%92%2C%E5%88%99%E6%9C%89%EF%BC%88%EF%BC%89A%EF%BC%8Ecos+a%EF%BC%9D4%2F5+B.cos+a%EF%BC%9D%EF%BC%8D4%2F5+C.cos+a%EF%BC%9D%C2%B14%2F5+D.cos+a%EF%BC%9D2%2F5)
3.设sin a=3/5,a为第II象限角,则有()A.cos a=4/5 B.cos a=-4/5 C.cos a=±4/5 D.cos a=2/5
3.设sin a=3/5,a为第II象限角,则有()A.cos a=4/5 B.cos a=-4/5 C.cos a=±4/5 D.cos a=2/5
3.设sin a=3/5,a为第II象限角,则有()A.cos a=4/5 B.cos a=-4/5 C.cos a=±4/5 D.cos a=2/5
B.cos a=-4/5
技巧
先根据勾股定理,直角边为三,斜边为五,则别一直角边为四,再判断cos第二项线为负,所以为4/5