Question 1128883
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It is WELL KNOWN identity.


If you want to learn the proof, look into the lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/The-proofs-of-the-formulas-for-arithmetic-progressions.lesson>The proofs of the formulas for arithmetic progressions</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/Problems-on-arithmetic-progressions.lesson>Problems on arithmetic progressions</A>  

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Sequences-and-series/Mathematical-induction-and-arithmetic-progressions.lesson>Mathematical induction and arithmetic progressions</A>

in this site.


Since your post does not contain any question, I can not say nothing more.



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For your info:


<pre>
    In this forum, the posts without question are considered as nonsensical.

    They are the subject of ignoring.

     I personally consider such posts as mockery.
</pre>


Have a nice night / morning / day / afternoon / week.