Question
For three sets A, B and C, show that. $\text{A}\subset\text{B}\Rightarrow\text{C}-\text{B}\subset\text{C}-\text{A}.$

Answer

Given $\text{A}\subset\text{B}$ To show: $\text{C} - \text{B }\subset​\text{ C} - \text{A}$ Let $\text{x}\in\text{C}-\text{B}$ $\Rightarrow \text{x}\in\text{C and x}\not\in\text{B}$ [by defination of C - B] $\Rightarrow \text{x}\in\text{C and x}\not\in\text{A}$ $[\because\text{A}\subset\text{B}]$ This can be seen by the venn diagram above $\Rightarrow \text{x}\in\text{C - A}$ [by defination of C - A] Thus $\text{x}\in\text{C - B}\Rightarrow\text{x}\in\text{C - A.}$ Thus is true for all $\text{x}\in\text{C}-\text{B}$ $\therefore\text{C - B}\subset\text{C - A}.$

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