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Ответ:

3)

[tex]S_{n} = \frac{2a_{1} + d (n - 1)}{2} \times n[/tex]

[tex] \frac{2a _{1} + ( - 2)(8 - 1)}{2} \times 8 = 64 \\ \frac{2a_{1} - 2 \times 7}{2} = 8 \\ 2a _{1} - 14 = 16 \\ 2a_{1} = 30 \\ a_{1} = 15[/tex]

4)

[tex] \frac{b_{n}}{b_{m}} = {q}^{n - m} [/tex]

[tex] \frac{81}{3} = {q}^{5 - 2} \\ {q}^{3} = 27 \\ q = \sqrt[3]{27} \\ q = 3[/tex]

[tex]b_{1} = \frac{b_{2} }{q} [/tex]

[tex]b_{1} = \frac{3}{3} = 1[/tex]

[tex]S_{n} = \frac{b_{1}( {q}^{n} - 1) }{q - 1} [/tex]

[tex]S _{6} = \frac{1( {3}^{6} - 1) }{3 - 1} \\ S_{6} = \frac{729 - 1}{3 - 1} \\ S_{6} = \frac{728}{2} \\ S _{6} = 364[/tex]