Not to snipe at anyone, but skills like this seem pretty essential in solving problems in general. Little tricks from linear algebra like magnitude (what this post concerns) and projections show up in interesting and useful places.

Additionally, while they /are/ harder to access, reading and understanding various proofs in Math can be an even more beautiful and enlightening experience than just seeing the practical result.

Maybe it's the difference between realizing that setf can be using on all the generalized variables and actually reading the source and seeing why. Math is full of clever hacks.