no 3 dan 4 , tolong bantu ya

3). U2 = 5 -----> a + b = 5

U4 + U6 = 28
a + 3b + a + 5b = 28
2a + 8b = 28

a + b = 5 dikali 2
2a + 8b = 28 dikali 1

2a + 2b = 10
2a + 8b = 28
------------------- --
-6b = -18
b = -18/-6 = 3

b = 3 ----> a + b = 5
a + 3 = 5
a = 5 - 3 = 2

U9 = a + (n - 1)b
U9 = 2 + (9-1). 3
U9 = 2 + 8 . 3
U9 = 2 + 24 = 26

4). U2 = 8 ----> a + b = 8
U4 = 14 -------> a + 3b = 14
Un = 23

a + b = 8
a + 3b = 14
---------------- --
-2b = -6
b = -6/-2 = 3

b = 3 -----> a + b = 8
a + b = 8
a + 3 = 8
a = 8 - 3 = 5

Un = 23

a + (n - 1)b = 23
5 + (n - 1).3 = 23
5 + 3n - 3 = 23
3n + 2 = 23
3n = 23 - 2
3n = 21
n = 21/3 = 7

Moga membantu