Can I say eight-gon, nine-gon and ten-gon instead of octa-, nona-, and deca-gon?

As a non-native English speaker I struggle with Greek prefixes. Am I allowed to use just normal English numbers in place of them? Is it natural? Or do I have to learn how those Greek prefixes work at the end of the day?

Solution 1:

Mathematicians do use this form for bigger numbers. The Wikipedia article Heptadecagon currently contains the phrase "a regular 51-gon, 85-gon or 255-gon and any regular n-gon with 2^h times as many sides".

And in that context, you may find mathematicians using the form for smaller numbers: in an article about polygons of different sizes, I would not be surprised to meet "5-gon" or "8-gon".

But outside mathematics, I've never heard anybody say "eight-gon" or any of the others.

Solution 2:

Short answer: no.

Longer answer: you will probably be understood, but people will think it's strange. Almost all words in English have roots in other languages. For these words, you have correctly identified Greek as the original language. Curiously, you have taken exception with the prefixes, but not the suffix: -gon, which is also Greek.

As a more interesting case, triangle uses the -angle suffix, which comes from Latin.
_{( tri- is both a Greek and Latin prefix.)}

Personal note:
In my travels as a mathematician, I admit that I have heard and used n-gon a lot. When a mathematical discussion involved specific instances of an n-gon, it was sometimes most natural to say things like eight-gon; especially when other, higher -gon were included in the discussion. Outside of the context of pure maths, it would be extremely unusual for anyone, even a mathematician, to say eight-gon.