Nonetheless, I am amazed that every textbook I have seen uses the "optimize a quadratic form on the unit ball" argument rather than this algebraic once. Lots of students don't remember multivariable calculus well, and existence of maxima of continuous functions on multidimensional bounded domains is complicated. Plus, I find a lot of students have trouble with an inductive process like getting one eigenvector and splitting off an orthogonal complement.
This argument is just shuffling algebra around, combined with the fact that a sum of squares is nonzero. It seems clearly easier to me.
The people over at Secret Blogging Seminar have a great post on a calculus-free proof of the spectral theorem. I have a group of (very) applied mathematical students, many of whom have never had a multivariate calculus course, so a proof like this will be great. From the post:
I came across an interesting article on Digg about how to save (or not) for your child's college education. Small spoiler:
I’ll argue that what the original $18,000 investment could have given your child over 18 years is more valuable, enjoyable and memory-making than one year of college ever could be.
Doing and teaching mathematics.