Question
For any two sets of A and B, prove that:$\text{A}'\cup\text{B}=\text{U}\Rightarrow\text{A}\subset\text{B.}$

Answer

We have $\text{A}'\cup\text{B}=\cap,$ the universal set
To show: $\text{A}\subset\text{B}$
Let, $\text{x}\in\text{A}$
$\Rightarrow\text{x}\not\in\text{A}'$ $[\because\text{A}\cap\text{A}'=\phi]$
$\because\text{x}\in\text{A and A}\subset\cup$
$\Rightarrow\text{x}\in\cup$
$\Rightarrow\text{x}\in(\text{A}'\cup\text{B})$ $[\because\cup=\text{A}'\cup\text{B}]$
But, $\text{x}\not\in\text{A}',$
$\therefore\text{ x}\in\text{B}$
Thus, $\text{x}\in\text{A}\Rightarrow\text{x}\in\text{B}$
This is true for all $\text{x}\in\text{A}$
$\therefore\text{ A}\subset\text{B.}$

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