Dik : suku ke-n dinyatakan dg Un U3 = -2 dan U6= 7 Dit : U13

Un = a + ( n - 1 ) b

U3 = a + 2b = - 2
U6 = a + 5b = 7
------------------------(-)
- 3b = - 9
b = 3

subtitusi
a + 2b = - 2
a + 2(3) = - 2
a + 6 = - 2
a = - 2 - 6
a = - 8

U13 = a + 12b
= - 8 + 12(3)
= - 8 + 36
= 28

U3 = a + 2b = -2
U6 = a + 5b = 7
      [ dikurangi, karena tandanya beda ]
               3b =  9
                 b = 3
                                     U3 = a + 2b = -2
                                             a + 2.3 = -2
                                             a + 6   = -2
                                             a = -2-6
                                             a = -8
U13 = a + [ n - 1 ] x b
          -8 + [ 13 - 1 ] x 3
          -8 + 12 x 3
            -8 + 36
               28