Question

Find S16 for 1 + 7 + 13 + 19 +

Answer 1

$S_n=\dfrac{a_1+a_n}{2}\cdot n\\n=16\\a_1=1\\a_{16}=?\\\\a_n=a_1+(n-1)\cdot d\\d=6\\a_{16}=1+(16-1)\cdot6=1+90=91\\\\S_{16}=\dfrac{1+91}{2}\cdot6=46\cdot6=276$