[latex]a_n=a_1+(n-1) cdot r \ left { {{a_1+2r=-5} atop {a_1+7r=5}} ight. \ (-)----\ -5r=-10/:(-5) \ r=2 \ a_1+2 cdot 2=-5 \ a_1+4=-5 \ a_1=-9 \ a_n=-9+(n-1) cdot 2 \ a_9=-9+8 cdot 2 \ a_9=-9+16 \ a_9=7[/latex]
Wzór: [latex] a_{n} = a_{1} +(n-1)r[/latex] [latex] a_{3} = a_{1} +(3-1)r = a_{1}+2r \ -5=a_{1}+2r[/latex] [latex]a_{8}=a_{1}+(8-1)r=a_{1}+7r \ 5=a_{1}+7r[/latex] Układ równań: [latex]left { {{-5=a_{1}+2r} atop {5=a_{1}+7r}} ight. \ left { {{5=-a_{1}-2r} atop {5=a_{1}+7r}} ight. \ 10=5r \ r=2 [/latex] [latex]a_{8} + r = a_{9} \ 5+2=7 \ a_{9}=7[/latex] r- różnica
