a1+(a1+d)+(a1+2d)+(a1+3d)+(a1+4d)=75
5a1+10d=75
a1+2d=15
2d=15-a1
2d=15-7
2d=8
d=4
a_(n-2)+a_(n-1)+a_(n)=129
a1+(n-3)d+a1+(n-2)d+a1+(n-1)d=129;
3a1+(n-3+n-2+n-1)d=129
3*7+(3n-6)*4=129
(3n-6)*4=108
3n-6=27
3n=27+6
3n=33
n=11
a_(11)=a1+10d=7+10*4=47
О т в е т. 47