This proof's use of "clear" is sorta reasonable, since they are probably assuming the reader is familiar with alternating series with decreasing terms. I would

This proof's use of "clear" is sorta reasonable, since they are probably assuming the reader is familiar with alternating series with decreasing terms. I would agree that it is sometimes fairly annoying. "Proof by intimidation" A: "it is clear that X" B: "Sorry, why is it clear?" A: "No, it is CLEAR that X!"