Terms and middle terms

You will write three small functions for binomial expansions: the general term, the value of a whole expansion, and the middle term numbers. The checker tests them on many cases.

The idea. The $(r + 1)$th term of $(a + b)^n$ is ${}^nC_r\,a^{n-r}b^r$. Adding all the terms from $r = 0$ to $r = n$ gives $(a + b)^n$. The middle term numbers are $\dfrac{n}{2} + 1$ for even $n$, and $\dfrac{n + 1}{2}$ and $\dfrac{n + 3}{2}$ for odd $n$.

Your tasks.

1. Finish general_term(n, r, a, b). Return the value of the $(r + 1)$th term.
2. Finish expansion_value(n, a, b). Add the general terms for r from 0 to n.
3. Finish middle_terms(n). Return a list with the middle term number or numbers, in order.