I don't understand how they got rid of the negative.(adsbygoogle = window.adsbygoogle || []).push({});

l = absolute, but they aren't used anyways.

[tex]\mid \S_n - frac{a}{1 - R} \mid = \mid a( \frac{1 - r^n}{1-r} ) - \frac{a}{1-r}\mid[/tex]

[tex]= \mid \frac{a r^n}{1-r} \mid[/tex]

I'm getting...

[tex]= \mid \frac{-a r^n}{1-r} \mid[/tex]

I always miss negatives, but I don't think I did this time.

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# Can someone double check this, or tell me happened?

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