In math we have certain values which are "able to be approximated". I have seen both approximatable and approximable. It would seem like approximatable is the correct version, but estimable means "able to be estimated". Neither approximatable nor approximable are in the OED. Is there any way to tell which should be correct?
In my view, both would be acceptable. Googling produces 67,300 pages using approximable, but only 161 pages using approximatable. The vast majority of all these pages seem to come from mathematics-related websites, so the prime users seem to prefer the former.
I, as a learner of English, am hesitant from using any word that I cannot find in both OED and Webster.
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