[tex]\it b_{11} = b_9\cdot q^2 \Longrightarrow q^2=\dfrac{b_{11}}{b_9} =\dfrac{3072}{768} =4[/tex]
[tex]\it q^2=4 \Longrightarrow q = \pm2[/tex]
[tex]\it b_1=\dfrac{b_9}{q^8} =\dfrac{768}{256} \Longrightarrow b_1= 3[/tex]
[tex]\it b_2 = b_1\cdot q[/tex]
[tex]\it b_2= 3\cdot(-2) =-6\ \ \ sau\ \ b_2 = 3\cdot2 =6[/tex]
