Suku ke 13 barisan 3,11/2, 8, 21/2,..., 98, .....

U1 = 3
U2 = 5.5
U3 = 8 

beda = U2 - U1 = 5.5 - 3
         = 2.5

U13 = a + 12 x beda
       = 3 + 12 x 2.5
       = 3 + 30
       = 33