tolong kerjakan pakai cara kerja

Un = 3n - 5
b = 3
U1 = 3.1 - 5
     = 3 - 5
     = -2

Sn = [tex] \frac{n}{2} (2U1 + (n-1)b) \\= \frac{n}{2} (2.-2 + (n-1)3) \\ = \frac{n}{2} (-4 + 3n-3)\\ = \frac{n}{2} (3n-7)[/tex]