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  1. Feb 2021
    1. 𝐡βˆͺ(β‹‚π‘–βˆˆπΌπ΄π‘–)=β‹‚π‘–βˆˆπΌπ΅βˆͺ𝐴𝑖Bβˆͺ(β‹‚i∈IAi)=β‹‚i∈IBβˆͺAiB \cup\left(\bigcap_{i \in I} A_{i}\right)=\bigcap_{i \in I} B \cup A_{i};

      Here's a simpler (hopefully similarly rigorous) proof I made for 1.1.3.a.

      $$\bigcup_{i \in I}A_{i} \cup B =(B \cup A_{1}) \cap (B \cup A_{2}) \cap ... \cap(B \cup A_{i})$$

      because distributive property of union over intersection $$= B \cup (A{1} \cap A{2}\cap ... \cap A_{i})$$

      $$= B \cup (\bigcap_{i \in I}A_{i})$$

    2. detain

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