for any two sets A and B prove that B is a subset of A union B  

Dear student,

Kindly find the answer to your query:
$$\begin{gathered} \mathrm{Let} \mathrm{A} = \left\{1,2,3\right\} \\ \mathrm{B} = \left\{7,8 \right\} \\ \mathrm{So} \mathrm{A} \cup \mathrm{B} = \left\{1,2,3,7,8\right\} \\ \mathrm{So} \mathrm{here} \mathrm{we} \mathrm{can} \mathrm{see} \mathrm{that} \mathrm{B} \mathrm{is} \mathrm{subset} \mathrm{of} \mathrm{A}\cup \mathrm{B} \end{gathered}$$

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