There is an arrow from "Do known techniques apply?", labelled "No". It leads to "Try to find a new technique". There's an arrow back from there to "Do known techniques apply?". Obviously, if you succeeded in finding a new technique applicable to the problem, the set of "known techniques" has expanded and you then follow the "Yes" arrow from "Do known techniques apply?".
Is it your opinion that there is something wrong with this? If so, what?
(If you respond with another passive-aggressive "Look at X" reply, I shall ignore it unless looking at X immediately convinces me that you've been right all along and I've been missing something.)
(2) It seemed obvious, and moreover too trivial a point to merit more than just pointing it out. I apologize if I've caused any offense.
* To clarify, I don't consider "find a new technique" to be a technique, which is why I said to look at the arrows.
Also consider the context. This was a handout to students in a first linear algebra class, that was meant to help them learn to do basic proofs on their homework problems. Nobody expected them to be engaged in original research. Any useful technique they needed to "discover" was very likely to be well-known to lots of people, including me.
Finally it is not clear to me why you think that the word "known" is superfluous. There is a world of difference between the stage where you are running through the techniques you know, trying to find one that fits, and the stage where you're engaged in expanding your list of available techniques. I was trying to get at that difference.
I was just saying that you can't apply a technique you don't know.