11. Lemma 2.1.21- Cauchy Integral Theorem for rectangles

Let $f : U \to C$ be a holomorphic function, and let $R = [a, b] \times [c, d]$ be a rectangle contained in $U$ . Then for any parametrization of $δ R$ , say $γ : [t_{0}, t_{1}] \to U$ , we have:
:?:

\int_{γ} f (z) d z := \int_{t_{0}}^{t_{1}} f (γ (t)) γ^{'} (t) d t = 0