Все на картинке
1) a_(18)=a_(1)+d*(18-1)=7+4*17=75 2)d=a_(2)-a_(1)=4-8=-4 a_(16)=a_(1)+d*(16-1)=8-4*15=-52 S_(16)=(a_(1)+a_(16))*16/2=(8-52)*8=- 44*8 = - 352 3)b_(7)=b_(1)*q^(6)=(-25)*(-1/5)^(6)=(-5)^2/(-5)^6=1/(-5)^4=1/(625) 4)S_(5)=b_(1)*(q^(5)-1)/(q-1)=11*(2^5-1)/(2-1)=11*31=341