`Có: u5 = 96`
` u6 = 192`
Mà: u6 = u5.q = 96.q
`⇒ 96.q = 192`
`⇔ q = 2`
Lại có: u5 = u1.$q^{4}$
⇔ u1 = $\frac{u5}{q^{4}}$
⇔ u1 = $\frac{96}{2^{4}}$
⇔ u1 = 6
⇒ u3 = u1.q² = 6.2² = 24
Vì q $\neq$ 1
=> S4 = $\frac{u1.(1-q^{4})}{1-q}$ = $\frac{6.(1-2^{4})}{1-2}$ = $\frac{-90}{-1}$ = 90
Vậy u3 = 24
S4 = 90