Odpowiedź:
[tex]S_{n}=\frac{2a_{1}+(n-1)r }{2}*n \\[/tex]
[tex]8=\frac{2*\frac{4}{5}+(n-1)*\frac{2}{5} }{2}*n[/tex] /*2
[tex]16=(\frac{8}{5}+\frac{2}{5}n-\frac{2}{5})*n[/tex]
[tex]16=\frac{6}{5}n+\frac{2}{5} n^{2}[/tex] /*5
80=6n+2[tex]n^{2}[/tex]
[tex]2n^{2}+6n-80=0[/tex]
Δ=[tex]6^{2}+4*2*(-80)=36+640=676[/tex]
[tex]n_{1} =\frac{-6-26}{4}=-8[/tex]
[tex]n_{2}=\frac{-6+26}{4}=5[/tex]
Odp. 5 wyrazów ciągu
