[tex]\displaystyle\bf\\a_{1} =2\\\\a_{8} =23\\\\S_{8} =?\\\\\\S_{8} =\frac{a_{1}+a_{8} }{2} \cdot 8=(a_{1} + a_{8} )\cdot 4=(2+23)\cdot 4=25\cdot 4=100\\\\\\Otvet \ : \ S_{8} =100[/tex]