题目 44.[数学[单选题]]-|||-,-|||-已知 sin alpha =dfrac (1)(2), 且α是第二象限的角,则cosα的值是 ()-|||-i OA. dfrac (1)(2)-|||-○B. dfrac (sqrt {2)}(2)-|||-C. dfrac (sqrt {3)}(2)-|||-D. -dfrac (sqrt {3)}(2) 题目解答答案