tolong dong besok mau dikumpul

2. beda = 18-15/9-3 = 3/6 = 1/2
   U3= a + (n-1)b = 15
          a + (2)1/2 = 15
            a + 1 = 15
             a = 15-1 = 14
U12 = a + (n-1)b
U12 = 14 + 11 . 1/2
U12 = 14 + 11/2
U12 = 28+11/2
u12 = 39/2 = 19 1/2
jawaban e