Question

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

Answer 1

а1=-2;              Sn=а1+аn/2 *n                                         a4=4+3=7
а2=1;                d=а2-а1=1-(-2)=3                                    a5=7+3=10
а3=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