55,51,47,43,39,35,31,27,23,19,15,11,7,3
55+3=58,51+7=58,47+11=58,43+15=58,39+19=58,35+23=58,31+27=58
58.7=406
-----
ili
a1=55, a14=3
s14=14/2.(a1+a14), s14=7.(55+3)=7.58=406
------
d=-4, an=a1+(n-1).d i eto nado bolše nulja.
55+(n-1)(-4) bolše čem 0
55-4n+4=59-4n bolše čem 0, 4n menše čem 59, poclednij člen progresii
imeet n=14
a14=a1+13d=55-13.4=55-52=3