Any 2 points determine a line,If there are 6 points in a plane,no 3 of which lie on the same line,how many lines are determined by pairs of these 6 points?
任何两点决定一直线.如果平面上有6个点,没有3个点在一条线上.这6个点一共可以决定多少直线.
因为没有3点共线,所以每个点都可以和剩下五个点决定直线.所以是 6*5/2 = 15
除以2是因为每条线算了两次