Ta có

`{(u_1 + u_5 = 51),(u_2 + u_6 = 102):}`

`{(u_1 + u_1q^4 = 51),(u_1q + u_1q^5 = 102):}`

`{(u_1(1 + q^4) = 51(1)),(u_1q(1 + q^4) = 102(2)):}`

Lấy `(2):(1)`

`=> (u_1q(1 + q^4))/(u_1(1 + q^4) ) = 102/51`

`=> q = 2`

Thay `q = 2` vào `(1)`

`=> u_1(1 + 2^2 ) = 51`

`=> u_1 = 3`

`=> bba` đúng

Ta cí

`u_4 = u_1.q^3 = 3.2^3 = 24`

`=> bb\b` sai