Variables A B C D: Prop.

Theorem ex3_1: A\/B-> B\/A.
intros.
destruct H.
right.
apply H.
left.
apply H.
Qed.

Theorem ex3_2: (A\/B)\/C -> A\/(B\/C).
intros.
destruct H.
destruct H.
left.
apply H.
right. left.
apply H.
right. right.
apply H.
Qed.

Theorem ex3_3: (A\/B->C) -> (A->C) /\ (B->C).
intros.
split.
intros.
apply H.
left.
apply H0.
intro.
apply H.
right.
apply H0.
Qed.