b4=b1*q³
b8=b1*q⁷
648:8=q⁴
q⁴= 81
q= 3 или q= -3
1) q=3
b1= 8/27
S5= b1(q⁵-1)/q-1= 8/27*(243-1)/3-1= 8*242/27*2= 968/27
2) q= -3
b1= -8/27
S5= -8/27*(-243-1)/-3-1= -8*(-244)/27*(-4)= -488/27
