Răspuns :
a8 = a1 + 7r = 2 - 35 = -33
Sn = [tex] \frac{(a1 + an)*n}{2} [/tex]
S8 = [tex] \frac{(2 - 33)*8}{2} [/tex] = (2-33)* 4 = -124
Sn = [tex] \frac{(a1 + an)*n}{2} [/tex]
S8 = [tex] \frac{(2 - 33)*8}{2} [/tex] = (2-33)* 4 = -124
[tex]\displaystyle a_1=2,~r=-5,~S_8=?~~~~~~~~~~~~~~~~~~~~~~~~~~~~\boxed{
S_n=\frac{2a_1+(n-1) \cdot r}{2} \cdot n} \\ S_8= \frac{2 \cdot 2+(8-1)
\cdot (-5)}{\not2} \cdot \not8 \\ \\ S_8=[4+7 \cdot (-5)] \cdot 4 \\
S_8=(4-35) \cdot 4 \\ S_8=-31 \cdot 4 \\ S_8= -124 [/tex]
