数学问题,快,过程,好的话追加分一,SN是等差数列an的前n项和,若SN=SM(N不等于M),则SM+N=?二,设等差数列的前N项和为SN,已知a3=12,S12>0,S13

Answer 1

Sn=Sm -> n*a1+d*n*(n-1)/2=m*a1+d*m*(m-1)/2
->(n-m)a1+d*(m+n-1)(n-m)/2=0 -> a1+d*(m+n-1)/2=0
->am+n/2=0 -> S(m+n)=(m+n)*am+n=0
a1=a3-2d=12-2d -> S12=12a1+d*12*11/2=144+42d>0
-> d>-24/7
S13=13a1+d*13*12/2=157+65d d0 a1+6d0,a7