THE DR. FOERSTER CONSPIRACY
First piece of evidence:
This is the color usage throughout the book (not the cover). This observation forced us to dig deeper.
Second piece of evidence: obvious typos
How do you even take the antiderivative of whatever Be clever! is? The obvious solution:
The space is a typo.
Therefore, you must take the antiderivative of Beclever!, with another problem wanting the antiderivative of Beclever!!!
The double factorial function?! This can only mean that Dr. Foerster is trying to send us a secret, hidden message.
Third piece of evidence: reduced yellow ink further on in the book
As the book progresses, less green colors are used. Already not using any red is suspicious; resorting to only blue colors must mean something!
Dr. Foerster typed the entire book in one day.
This is the only logically sane solution to this problem. By typing the entire book in one day, he must have gotten so tired that he kept making mistakes.
Fourth piece of evidence: interestingly and specifically worded problems
Some problems are a bit too real-world. Therefore, some secret messages must be hidden.
Dr. Foerster is being held captive!
That's the only logical, sane, and trustworthy answer that can come from this obvious evidence.