Hi, I am just working on random problems to study for my midterm, but then I came across this one I don't know how to do. Plz help me. Thankkk youuu sooo much.

1) Suppose that A, B, C are collinear points in R^3 whose coordinates are given by (a1, a2, a3), (b1, b2, b3), and (c1, c2, c3) respectively. Prove that A*B*C holds if we have a1 < b1 < c1, a2 = b2 = c2, and a3 > b3 > c3.

Many thanks.

Jen