Mathematics are my friend should definitely be accepted, both are correct.
and perhaps "mathematics are my friends" as well
This is a strange sentence to me. Does it just mean that they really like Maths?
Have you not encountered such, no matter the language?
Mengapa tak? Saya mempikir bahwa guru matematika kalian tidak sudah mengajar dengan bagus
No. Math is not my friend. History is.
I actually kinda miss doing proofs and diff eqs