Final answer:
The Addition Property justifies the given prove statement that AD + CD = DE + DE.
Step-by-step explanation:
The property that justifies the given prove statement is the Addition Property.
This property states that if two quantities are equal, then adding the same value to both sides of an equation will still result in two equal quantities.
In this case, since we are given that AD = DB and CD = DE, we can use the Addition Property to add AD and CD to both sides of the equation to prove that AD + CD = DE + DE.