We have geometric Hahn Banach theorem in standard functional analysis course, saying that there exists a separating hyperplane separates two special sets in usual topology. However this theorem can be used for some special sets in weak* topology. This is the problem 9 in the functional analysis book by Prof. Brezis.
Theorem 1 Let be a Banach space, be two nonempty disjoint convex subsets. Assume is open in weak* topology. Then there exist some and a constant such that the hyperplane separates and .