Are “where” subclauses acceptable?

Solution 1:

Yes. Subordinate clauses that start with "where" are more than acceptable; they're completely proper, both grammatically and semantically. By placing the "where" and its clause between commas, one is creating an non-restrictive clause, which is to say that one is providing parenthetical information that is not necessary to the operation of the sentence, information that one could scoop out of the sentence and still have the sentence make sense and mean the same thing. Essentially, it's proverbially putting one's hand by one's mouth and saying, "By the way..."

Are there better choices instead of this available? That's a judgment call and entirely depends on the situation. Clearly, in mathematics, as you've demonstrated, the overall opinion is that this particular construction is the best way to convey such information, a way that is clear and concise and so has become prolific in writing mathematical problems.