Tolong dibantu yaa teman teman

diket : U5 = 11 ; U8 + U12 = 52
dit : S8 = ....
jawab :
U5 = a + 4b = 11 ... pers. (1)
U8 + U12 = (a + 7b) + (a + 11b) = 52
              = 2a + 18b = 52 ... pers. (2)

eliminasi a dari pers. (1) dan (2)
a + 4b = 11     --> 2a + 8b = 22
2a + 18b = 52 --> 2a + 18b = 52
                           -------------------- -
                                  - 10b = - 30
                                        b = 3

cari a (suku pertama) :
a + 4b = 11 --> a + 4(3) = 11 --> a = -1

cari S8 :
S8 = (8/2) (2(-1) + (7)(3))
     = (4)(-2 + 21)
     = (4)(19) = 76