I know that English pronunciation is rather arbitrary. There are still some "rules" that even with many exceptions are useful for non-natives like me.
I'm puzzled about the pronunciation of the very common prefix "tri", meaning "three".
"Tri" is often pronounced /traɪ/. For example: triangle, trilateral, tripartite, trioxide, triode, triad, trilobites, triglyceride, triennium, tridimensional, trigeminal, tricycle, tricuspid, triceps, triathlon.
But sometimes "tri" is pronounced /trɪ/. For example: trigamous, triplicate, trio, trinity, trilogy, trillion, trigonometry, triplet, triple, triphthong, triploid, triptych.
Finally, it seems that a few words can be pronounced both ways. For example: trimester, tricolor.
I can find no pattern at all. Is there any "rule"? Or can you help me in any way to guess the right pronunciation of a new word with the prefix "tri"?