Odpowiedź:
[tex]a_{7} = 26[/tex]
[tex]S_{12}[/tex] = 294
Szczegółowe wyjaśnienie:
[tex]a_{1} =8 , r = 3[/tex]
[tex]a_{n} = a_{1} +(n-1)*r[/tex]
[tex]a_{7} = 8 +(7-1)*3=8+6*3=8+18 = 26[/tex]
[tex]a_{12} = 8 +(12-1)*3=8+11*3=8+33 = 41[/tex]
[tex]S_{n} =\frac{a_{1}+a_{n} }{2} *n[/tex]
[tex]S_{12} =\frac{8+41 }{2} *12[/tex] = [tex]\frac{49}{2} *12[/tex] = 294
