(* Lemma RInt_comp_opp (f : R -> V) (a b : R) : ex_RInt f (-a) (-b) -> RInt (fun y => f (- y)) a b = opp (RInt f (-a) (-b)). *)

Lemma RInt_opp (f : R -> V) (a b : R) :
ex_RInt f a b ->
RInt (fun x => opp (f x)) a b = opp (RInt f a b).