Question 142143
The sum of the first n terms of an arithmetic progression is {{{na+(n(n-1)r)/2}}}.

n=12    <--- # of terms
a=-5  <--starting term
r=5  <--- common difference between terms


s={{{12*(-5)+(12(12-1)5)/2}}}
={{{-60+330=270}}}


We can check by iterating every term:

-5+0+5+10+15+20+25+30+35+40+45+50=270.