${{S}_{17}}=221\Rightarrow \frac{17({{a}_{1}}+{{a}_{17}})}{2}=221\Rightarrow {{a}_{1}}+{{a}_{17}}=26$ می‌دانیم: ${{a}_{1}}+{{a}_{17}}={{a}_{5}}+{{a}_{13}}={{a}_{8}}+{{a}_{10}}\Rightarrow {{a}_{5}}+{{a}_{13}}+{{a}_{8}}+{{a}_{10}}=26+26=52$