【How-to】What does the upside down u mean in math

∪ The symbol ∪ means union. Given two sets S and T, S ∪ T is used to denote the set {x|x ∈ S or x ∈ T}. For example {1,2,3}∪{3,4,5} = {1,2,3,4,5}. … \ The symbol \ means remove from a set. Given two sets S and T, S\T is used to denote the set {x|x ∈ S and x /∈ T}.