题目例1(1)(2021·全国甲卷)若 alpha in (0,dfrac (pi )(2)), tan2α-|||-=dfrac (cos alpha )(2-sin alpha ) 则tanα等于 ()-|||-A. dfrac (sqrt {15)}(15) B. dfrac (sqrt {5)}(5) C. dfrac (sqrt {5)}(3) D. dfrac (sqrt {15)}(3)题目解答答案