If $f : U \rightarrow \mathbb{C}$ is differentiable in $U$ open subset of $ \mathbb{C}$ then $f$ is analytic in $U$.
(In the proof you consider a triangle $T_1$ in $U$, subdivide it in a sequence of smaller triangles $T_n $, observe that $ \displaystyle \bigcap_{n=1} ^{\infty} T_n = \lbrace z_0 \rbrace $, show that $ \displaystyle \int _{\partial T_1} f(z) dz = 0 $ and then apply Morera's theorem).
At a certain point in the proof, since $f$ is differentiable it is stated correctly that $ |f(z) - f(z_0) - f'(z_0)(z-z_0)| \leq \varepsilon_n (z-z_0)$ with $\varepsilon_n \rightarrow 0$ as $n \rightarrow \infty$; then $ \left| \int_{\partial T_n} f(z) dz \right| = \left| \int_{\partial T_n} f(z) - f(z_0) - f'(z_0)(z-z_0) dz \right| $ How is this obtained?