Is Duo mispronouncing "petryal" here? The audio makes the "y" sound like "uh" but on Gweiadur it sounds like "ee" as if it were a final syllable.
I would go with either one here. It is an unusual word and not one that is likely to have been sampled when Ivona.com created the sound database for Welsh, I suspect. Either pronunciation is likely to be recognised, though.
in my dictionary petryal is a square
As an adjective, petryal means square as in square/right-angled. As a noun, it means a rectangle or any figure with square/right angles
A square (sgwâr) is just a special case of rectangle.