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Please tell me how to start on this proof or give me some kind of hint. Please click on this link to see the question

Show that if $X_1,X_2,\dots,X_n$ denotes an iid sample from $N(\mu,\sigma^2)$ and $\sigma^2$ is known, then the best $(1-\alpha)100\%$ confidence interval is $\Big(\bar{x} \mp z_\tfrac{\alpha}{2}\frac{\sigma}{\sqrt{n}}\Big).$

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Hint: $$ \frac{\bar{X}-\mu}{\sigma/\sqrt{n}}\sim\mathcal{N}(0,1), $$ where $\bar{X}=\frac{1}{n}\sum_{i=1}^n X_i$.

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