bantu sama caranya ya makasi 2 nomor

89. 20, 17, 14, 11, 8, ...
U1 = a = 20
b = U2 - U1 = 17 - 20 = -3
U50 = a + (n - 1)b
= 20 + (50 - 1)-3
= 20 + (49)-3
= 20 + (-147)
= -127
=> jawaban yang benar (B) ✔

90. 2, 4, 8, 16, ...
U1 = a = 2
[tex]r = \frac{u2}{u1} = \frac{4}{2} = 2[/tex]

[tex]un \: = a \times r {}^{n - 1} \\ \: \: \: \: \: \: \: \: = 2 \times 2 {}^{n - 1} \\ \: \: \: \: \: \: \: \: = 2 {}^{2n - 1} [/tex]
=> jawaban yang benar (D)