题目

You’d better ________ the piano now. Your father is sleeping.             A. not to play         B. not play      C. don’t play        D. not playing 答案：B已知角α、β的顶点在坐标原点，始边与x轴的正半轴重合，α、β∈（0，π），角β的终边与单位圆交点的横坐标是-13，角α+β的终边与单位圆交点的纵坐标是45，则cosα= ．