**My first post is about two properties in the elementary set theory. You can see an amazing proof for them from me in the following.
**

**In proof I used the following property in logic**

**that is called “constructive dilemma”.**

**First Property:**

**Let** **, then I can write
**

**which implies that** **.**

**Second Property:**

**Let** **, then**

**which implies that** **.**