b_(5)=b_(1)*q^(4),
81=16*q^(4),
q^(4)=81/16,
q^(4)=(3/2)^(4),
q= ± (3/2).
Если q=-(3/2), то:
b_(2)=16*(-3/2)=-24,
b_(3)=-24*(-3/2)=36,
b_(4)=36*(-3/2)=-54.
Eсли q=3/2, то:
b_(2)=16*(3/2)=24,
b_(3)=24*(3/2)=36,
b_(4)=36*(3/2)=54.
Ответ: -24; 36; -54 или 24; 36; 54.