Question
For any two sets A and B, prove the following:
$\text{A}\cap\text{(A}\cup\text{B})=\phi$

Answer

$\text{LHS}=\text{A}\cap\text{(A}\cup\text{B}')$
$=\text{A}\cap\text{(A}'\cap\text{B}')$ [By De-morgan's law]
$=\text{(A}\cap\text{A}')\cap\text{B}'$ [By associative law]
$= \oint \cap \text{ B}'$ $[\because \text{A}\cap{A}' =\oint]$
$= \oint$
$= \text{RHS}$
$\therefore$ LHS = RHS Proved.

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