$1.$e$.$ What is a Regular Expression?

Write regular expressions for:

i$)$ The language consising of strings that start and end with different symbol. ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}$

ii$)L=\{\begin{array}{c}{a}^n{b}^m{c}^k\text{:}n+m\end{array}$ is odd and $k$ is even