Prove: If C⊂A and D⊂B then D−A⊂B−C
Let \(x\in D\setminus A\), so that \(x\in D\) but \(x ot\in A\). Since \(D\subset B\), it follows that \(x\in B\), and since \(C\subset A\), it follows that \(x ot\in C\), which means \(x\in B\setminus C\).
