题目

Children with single parents at my school were very much in the (少数). 答案：【1】minority已知△ABC顶点的直角坐标分别是A（3，5）、B（0，1）、C（8，-7）．（1）求cosB的值；（2）若AD=（-2，-5），证明：B、C、D三点共线．