Question 849495
<pre>
I changed my solution:

{{{sum((-3-4i),i=3,n)}}}

That's the sequence

-3-4(3) = -3-12 = -15
-3-4(4) = -3-16 = -19
-3-4(5) = -3-20 = -23
-3-4(6) = -3-24 = -27
...
-3-4(n) = -3-4n

So that is the sum of the arithmetic sequence with
first term -15, last term -3-4n and n-2 terms [there are
2 less since are no terms for i=1 or i=2]

Sum = {{{expr((number_of_terms)/2)(first_term+last_term)}}}

Sum = {{{expr((n-2)/2)(-15+(-3-4n))}}}

Sum = {{{expr((n-2)/2)(-15-3-4n)}}}

Sum = {{{expr((n-2)/2)(-18-4n))}}}

Sum = {{{expr((n-2)/2)(-2)(9+2n))}}}

Sum = {{{expr((n-2)/cross(2))(-cross(2))(9+2n))}}}

Sum = {{{-(n-2)(9+2n))}}}

Edwin</pre>