Final answer:
In an ERD using Chen notation, a weak entity is depicted by a double rectangular box. It represents an entity that cannot exist without a corresponding strong entity. A strong entity is represented by a single rectangle, and primary keys are underlined within their entity.
Step-by-step explanation:
In an Entity-Relationship Diagram (ERD) using Chen notation, a weak entity is signified by a double rectangular box. Weak entities are those that cannot exist without the presence of a strong entity they are related to. Their existence is dependent on another entity, referred to as the owning or parent entity. To illustrate, consider a database of a library. A book copy could be a weak entity because it can't exist without the book it is a copy of. Hence, a book copy would be represented by a double rectangle in an ERD.
Strong entities, on the other hand, are self-existing and do not depend on other entities for their existence. A strong entity would simply be depicted by a single rectangle in Chen's notation. Strong relationships are shown using a diamond shape, and primary keys are not represented by a specific shape but are underlined within the entity that they belong to.