Prove that A is a subset of phi implies A=phi

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$$\begin{gathered} \mathrm{We} \mathrm{know} \mathrm{that} \varnothing \mathrm{is} \mathrm{subset} \mathrm{of} \mathrm{every} \mathrm{set}. \\ \mathrm{Also} \mathrm{number} \mathrm{of} \mathrm{subset} \mathrm{of} \mathrm{any} \mathrm{given} \mathrm{set} =2^{\mathrm{n}} \\ \mathrm{where} \text{'}\mathrm{n}\text{'} \mathrm{is} \mathrm{cardinal} \mathrm{number}. \\ \mathrm{For} \mathrm{empty} \mathrm{set} \mathrm{n}=0 \\ \mathrm{Number} \mathrm{of} \mathrm{subsets} \mathrm{of} \mathrm{empty} \mathrm{set} =2^0=1 \\ \mathrm{Hence} \mathrm{if} \mathrm{A} \mathrm{is} \mathrm{a} \mathrm{subset} \mathrm{of} \varnothing , \mathrm{then} \mathrm{A}=\varnothing \end{gathered}$$

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