Question 1043122
.
Find the Sn of the given {{{highlight(cross(artithmetic))}}} arithmetic series.
a1 = -100, d = -1, n = 21
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<pre>
{{{S[n]}}} = {{{(a[1] + (n-1)*d/2)*n}}} = {{{(-100 + (21-1)*(-1)/2)*21}}} = .?.


.?. means "calculate on your own".
</pre>

On arithmetic progressions read the lesson 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Sequences-and-series/Arithmetic-progressions.lesson>Arithmetic progressions</A> 

in this site and the lessons associated with it.