Question 612242
  <pre><font face = "Tohoma" size = 3 color = "indigo"><b> 
Hi, 
find the sum of each arithmetic series:  
Note: {{{a[n]=a[1]+(n-1)*d}}}  and {{{S[n]=(n(a[1]+a[n]))/2}}}
 {{{highlight(-4)}}} +1+6+...+ {{{highlight(91)}}}    || d= 5 
91 = -4 + (n-1)5 
n = 20 
{{{S[n]=(n(-4+ 91))/2= (87n)/2 = 870}}} 
13+20+27+...+272  ||d = 7
89+86+83+...+20   ||d = -3
8+11+14+...+23    ||d = 3