$2 . \mathrm{I} . 7 \mathrm{E} \quad$

A complex function is defined for every $z \in V$, where $V$ is a non-empty open subset of $\mathbb{C}$, and it possesses a derivative at every $z \in V$. Commencing from a formal definition of derivative, deduce the Cauchy-Riemann equations.

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