# Variance of Autoregressive models, AR(1)

I'm in high school writing an exam porject in which i have to present tommorow.

There is a proof of how you can calculate variance in an AR(1) mode that I just can't seem to understand. Could anyone please ELI5? Thank you so much.

So, my question is:

Picture

Where does the formula derive from? The long one, and how does it turn into the one under with the 0'es?

Thanks again

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