Answer
$(a + b)^n = \sum_{k=0}^n \binom{n}{k} a^{n-k} b^k$, where $\binom{n}{k} = \frac{n!}{k!(n - k)!}$.
Explanation
The general $(k+1)$-th term in the expansion is $T_{k+1} = \binom{n}{k} a^{n-k} b^k$, frequently tested in Gaokao algebra questions.