barisan geometri tersebut adalah

[tex]u2 = 4 \\ u4 = 16 \\ b = \frac{16 - 4}{4 - 2} = \frac{12}{2} = 6 \\ un = a + (n - 1)b \\ 4 = a + (2 - 1)6 \\ 4 = a + (1 \times 6) \\ 4 = a + 6 \\ a = 4 - 6 = - 2 \\ u8 = a + (n - 1)b \\ = - 2 + (8 - 1)6 \\ = - 2 + (7 \times 6) \\ = - 2 + 42 \\ = 40[/tex]