intros a b ha hb
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
⊢ ∀ (a b : α), a ∈ H → b ∈ H → a * b ∈ H
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
a b : α
ha : a ∈ H
hb : b ∈ H
⊢ a * b ∈ H