Różnica
[tex]a_8 + 2r = a_{10} \\ 0.6 + 2r = - 1 \\ 2r = - 1.6 | \div 2 \\ r = - 0.8[/tex]
Pierwszy wyraz
[tex]a_1 + 7r = a_8 \\ a_1 + 7( - 0.8) = 0.6 \\ a_1 + (- 5.6) = 0.6 \\ a_1 = 6.2[/tex]
Dwudziesty wyraz
[tex]a_1 + 19r = a_{20} \\ 6.2 + 19( - 0.8) = a_{20} \\ 6.2 + ( - 15.2) = a_{20} \\ a_{20} = - 9[/tex]
Suma
[tex]S_{20} = \frac{a_1 + a_{20}}{2} \times 20 \\ \\ S_{20} = \frac{6.2 +( - 9) }{2} \times 20 \\ \\ S_{20} = \frac{ - 2.8}{2} \times 20 \\ \\ S_{20} = - 1.4 \times 20 \\ \\ \boxed{S_{20} = - 28}[/tex]
