Дана арифметическая прогрессия -2,1,4 найдите сумму членов этой прогрессии с четвертого по одиннадцатый

а1=-2;              Sn=а1+аn/2 *n                                         a4=4+3=7а2=1;                d=а2-а1=1-(-2)=3                                    a5=7+3=10а3=4                  sn=-2+72 *4=10                                     a6=10+3=13                          sn=-2+102 *5=20                                  a7=13+3=16                          sn=-2+132 *6=33                                  a8=16+3=19                          sn=-2+162 *7=49                                  a9=19+3=22                          sn=-2+192 *8= 68                                 a10=22+3=25                          sn=-2+222 *9=90                                   a11=25+3=28                         sn=-2+252 *10=115                         sn=-2+282 *11=143