... which can drive you crazy - especially if you are not able to follow the writer's argument. One of these sentences is: "... that is trivially true."

Obviously, the writer finds it trivial, but maybe not the reader. Fortunately those sentences are rather rare in the material of the Open University, but now in level 3, particularly in the Number Theory course, this sentence appears more and more. Even though it sometimes can drive me crazy, I still appreciate it that the Open University uses it, because they use it with caution and it is, like other things introduced little by little.  

This was really different at the Fernuni Hagen. Already in the first course booklets they bombard the reader with such terms and assume many knowledge which is no longer taught at school.

But what does "trivially true" mean?

The word "trivial" originates from the Latin word "Trivium" which means "the three ways". At mediaeval universities the students had to complete courses in grammar, logic and dialectics and rhetoric before they were able to start studying their "subject". Hence these basics were then trivial.

Nowadays the word "trivial" describes properties or arguments, which follow directly from a definition or theorem. But this also depends from the writer and reader's knowledge and it also depends from the mathematical subjec. For instance, the equation 1+1=2 is trivial in number theory, but not in set theory.

(Beutelspacher, Albert; Das ist o.B.d.A. trivial; Gießen, 2008, p. 41f.)