Jawaban:

1. u6 = 20 dan u9 = 38

u6 = a + 5b = 20

u9 = a + 8b = 38 -

-3b = -18

b = -18/-3

b = 6

u6 = a + 5b = 20

u6 = a + 5(6) = 20

u6 = a + 30 = 20

a = 20 - 30

a = -10

maka u25

u25 = a + 24b

u25 = -10 + 24(6)

u25 = -10 + 144

u25 = 134

2. u8 = 7 dan u12 = 19

u8 = a + 7b = 7

u12 = a + 11b = 19 -

-4b = -12

b = -12/-4

b = 3

u8 = a + 7b = 7

u8 = a + 7(3) = 7

u8 = a + 21 = 7

a = 7 - 21

a = -14

maka suku pertama nya adalah -14 dan bedanya adalah 3

3. suku pertamanya adalah 1

rasionya adalah 4/1 = 4

u9 = a.r^(n-1)

u9 = 1.4⁸

u9 = 65.536

4. u3 = 14

u7 = 30

u3 = a + 2b = 14

u7 = a + 6b = 30 -

- 4b = -16

b = -16/-4

b = 4

u3 = a + 2b = 14

u3 = a + 2(4) = 14

u3 = a + 8 = 14

a = 14 - 8

a = 6

maka u20 = a + 19b

u20 = 6 + 19(4)

u20 = 6 + 76

u20 = 82

5.

diketahui a = 27 dan r = 9/27 = ⅓

(a) rumus suku ke-n

un = a.r⁽ⁿ⁻¹⁾

un = 27.⅓⁽ⁿ⁻¹⁾

un = 3³·3⁻¹⁽ⁿ⁻¹⁾

un =3³·3⁻ⁿ⁺¹

un = 3³⁻ⁿ⁺¹

un = 3⁴⁻ⁿ

(b) suku ke 8

u8 = 3⁴⁻⁷

u8 = 3⁻³

u8 = 1/3³

u8 = ¹/₂₇