A set A of integers is called an ideal if and only ifi. 0 ∈ A,ii. if a ∈ A, then also −a ∈...

A set A of integers is called an ideal if and only if

i. 0 ∈ A,

ii. if a ∈ A, then also −a ∈ A, and

iii. il a, b ∈ A, then a + b ∈ A.

[BB] Prove that for every integer n ≥ 2 the number of lines obtained by joining n distinct points in the plane, no three of which are collinear, is