A:how is it that
B:what is it that
C:how it is that
D:what it is that
向量OA→=(k,12),OB→=(4,5),OC→=(10,k),当k为何值时,A,B,C三点共线?
二次函数y=﹣x2+2x﹣3图象的顶点坐标是____.