Let $z_1 = a+bi, z_2 = c+di, z_3 = e+fi \in \mathbb{C}$, where $a, b, c, d, e, f \in \mathbb{R}$, and let $r, s \in \mathbb{R}$.
1. Closure under addition: $z_1 + z_2 = (a+bi) + (c+di) = (a+c) + (b+d)i$. Since $a+c \in \mathbb{R}$ and $b+d \in \mathbb{R}$, $z_1
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