Why does $L\subseteq \textbf{P} \cap \textbf{NP}$ is $\textbf{NP}$-complete imply $\textbf{NP} = \textbf{P}$? [duplicate]

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If I show that a language $$L$$ is contained in $$\textbf{P}$$ and $$\textbf{NP}$$ and I know that the language is $$\textbf{NP}$$-complete, why did I proof that $$\textbf{P} = \textbf{NP}$$?

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Because you have then showed that $$L$$ is an $$\textbf{NP}$$-complete language which, since $$L \in \textbf{P}$$, is decidable in poly-time. Since any other language $$L' \in \textbf{NP}$$ is efficiently reducible to $$L$$ (because of $$\textbf{NP}$$-completeness), $$L' \in \textbf{P}$$ as well. It follows that $$\textbf{NP} \subseteq \textbf{P}$$ (and the other inclusion is trivial).