Question

÷ a1, a2, a3, 5, 8, 11. Calculați a1, a2, S8.

Answer 1

[tex]\it r = a_5-a_4 = 8-5=3 \\\;\\ a_1 = a_4-3r=5-3\cdot3=5-9=-4 \\\;\\a_2=a_1+r=-4+3=-1[/tex]

[tex]\it a_8=a_1+7r=-4+7\cdot3=-4+21=17 \\\;\\ S_8=\dfrac{(a_1+a_8)\cdot8}{2} = (-4+17)\cdot4= 13\cdot4= 52.[/tex]