minta tolong ya kerjain no 9-14 pake cara juga ya...

9.b = 54-62 = -8

un = a.( n-1 ). b
= 62.( n-1 ) .(-8).
= 62. -8n + 8
= -8n.70
10.U10 = 45
( a + 9b ) = 45...pers.1

U4 + u6 = 40
( a+3b) (a+5b) = 40
2a + 8b = 40....... pers.2

eliminasi pers.1 dan 2

a + 9b = 45
2a + 8b = 40

yg atas kali 2 yg bawah kali 1

maka

2a + 18b = 90
2a + 8b = 40

coret 2a

jadi di dapat
10b = 50
b = 5

substitusi nilai b

a + 9b = 45
a + 9(5)=45
a = 0

maka U15

a + 14b = 0 + 14(5)
= 70

12.U3 = 8
a + (3 - 1)b = 8
a + 2b = 8 ...(1)

U8 = 23
a + (8 - 1)b = 23
a + 7b = 23 ...(2)

eliminasikan (1) dan (2)
a + 2b = 8
a + 7b = 23
_________ -
-5b = -15
b = -15/-5 = 3

masukkan b = 3 ke persamaan (1)
a + 2b = 8
a + 2(3) = 8
a + 6 = 8
a = 8 - 6
a = 2

S5 = 5/2 (2a + (5 - 1)b)
= 5/2 (2(2) + (4)(3))
= 5/2 (4 + 12)
= 5/2 × 16
= 5 × 8 = 40

11.U4 = 7                          

U6 + U8 = 23
U4 + 2b + U4 + 4b = 23
2 U4 + 6b = 23
2 . 7 + 6b = 23
14 + 6b = 23
6b = 9
b = 1,5

U20 = U4 + 16b
       = 7 + 16 . 1,5
       = 7 + 24
       = 31

13.u2=7000
u10=9000
s12=...?

u2= a + b
7000= a + b
u10= a + 9b
9000= a+9b

63000=9a+9b
9000=a+9b
---------------------- -
54000=8a
6750=a

7000=a+b
=6750+b
250 = b

s12= 6 (2.6750+11.250)
=6(13500+2750)
=6(16250)
=97500

12 sama 13 mlz gk ketemu, semoga membantu dan sorry klau gambar gk jelas