Question

a1=116 ; a8=137. Calculati r=? ; s8 ..

Answer 1

$\displaystyle a_1=116,a_8=137,r=?,S_8=? \\ a_8=137 \Rightarrow a_{8-1}+r=137 \Rightarrow a_7+r=137 \Rightarrow a_1+7r=137 \Rightarrow \\ \Rightarrow 116+7r=137 \Rightarrow 7r=137-116 \Rightarrow 7r=21 \Rightarrow r= \frac{21}{7} \Rightarrow r=3 \\ S_8= \frac{232+7 \cdot 3}{2} \cdot 8 \Rightarrow S_8= \frac{232+21}{2} \cdot 8 \Rightarrow S_8= \frac{253}{2} \cdot 8 \Rightarrow \\ \Rightarrow S_8=253 \cdot 4 \Rightarrow S_8=1012$