You are right.
I can see it in the Skill itself and in the "Duolingo for schools" section about Math, that only these words are available:
plus......................Adverb
minus..................Preposition
gleich..................Adjective
geteilt durch.....Phrase

Sources:
https://www.duolingo.com/skill/de/Math
https://schools.duolingo.com/classroom/1851808/curriculum

You can report it to course moderator MultiLinguAlex

• via his stream (if you still have that possibility)
• or via a sentence discussion in which MultiLinguAlex wrote a comment (e.g in https://www.duolingo.com/comment/12816907)