formula untuk 2+7+12+17+22+...+(5n-3)

[tex]S_n= \frac{n}{2}(u_1+u_n)\\ = \frac{n}{2}(2+5n-3)\\ = \frac{n}{2}(5n-1)\\ = \frac{(5n^2-n)}{2}\\ [/tex]