triangular arbitrage and implied cross rates question examples, question, something, set

d2rockstar

Quick question, on p.489 in CFAi book in the second paragraph (2nd arbitrage constraint) they set the ratios so that to get a/c u need to have a/b and c/b….. but in all the examples its usually set up so that the relationship is a/c=(a/b)(b/c) and u can cancel out currency b (this is for questions without inverting) …. so  my question is: is the paragraph wrong? Or am i missing something? Sorry for any typos im on my tablet and it sucks to type