题目

 Although we have learned English for 3 years, we still have a little trouble_______ English grammar.     A. learn               B. learned                    C. learning               D. to learn    答案：C 给出下列四个命题，其中正确的命题有________①函数有一条对称轴方程是；②函数，可改写成；③若f（sinx）=cos6x，则f（cos15°）=0；④正弦函数在第一象限为增函数．