Many examples demonstrate the usefulness of technology and its integration into education curriculums. He is a strong supporter of the use of GeoGebra software for teaching and learning math, for which he has created more than 50 GeoGebra tutorial lessons and posted them on his site. These lessons start with the basics of introducing the software and how to use it, and then gradually advance to demonstrate how it can be used for both teaching and learning.
Another one of his most popular posts includes a list of more than 20,000 mathematics contest problems and solutions. This is an incredibly useful resource for anyone looking for practice questions. The problem sets come from various contests or past exams, so it is all very relevant material, and there is such a huge selection that you would be an expert if you managed to get through all of it!
Guillermo is a very knowledgable instructor, which is evident by the diverse selection and depth of his explanations and content. His blog has been created as a hobby, but you can see his passion for the subject in his writings. It is highly recommended that you pay his site a visit to see if you can find anything there that is helpful to you!