Just out of curiosity...
I always say "dee-why dee-ex" Some people say "dee-why by dee-ex" (especially many of my maths lecturers) Others say "dee-why on/over dee-ex"
I've noticed that most of my maths lecturers never say "$dy$ on $dx$" or "$dy$ over $dx$", not to mention some of my maths teachers in high school said that such a pronunciation is not correct. Is there a reason behind this? Is there a "correct" way to pronounce $\frac{dy}{dx}$?