Final answer:
The statement 'books are rectangular' is considered a prior, based on general knowledge and experience, before new evidence is introduced. It's used as a foundational belief in evaluating probabilities, such as calculating the likelihood of purchasing different types of books, where the shape might be a consideration in the system of interest.
Step-by-step explanation:
The statement "books are rectangular" could be considered as regarding prior probability, which reflects a belief or assumption about the probability of an event before new evidence is considered. In contrast, a likelihood refers to how probable some evidence is given an assumption or hypothesis. For instance, saying that "books are rectangular" is a prior belief that books generally have a rectangular shape. This does not require immediate evidence, but instead, it's based on our general experience and knowledge about books. When making inferences or predictions, such as in the context of a probability exercise like "Try It Σ 3.30" which involves calculating the probability of buying different types of books, the notion of rectangular books could inform our system of interest when we are considering the physical properties of books as compared to other objects or products.