b[1]=16
b[5]=81
b[n]=b[1]*q^(n-1)
b[5]=b[1]*q^4
81=16*q^4
q^4=81/16
q^2=9/4
q1=3/2 или q2=-3/2
b[2]=b[1]*q, b[3]=b[2]*q, b[4]=b[3]*q
b[2]=16*3/2=24, b[3]=24*3/2=36, b[4]=36*3/2=54
или
b[2]=b[1]*q, b[3]=b[2]*q, b[4]=b[3]*q
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