1.Pada suatu barisam aritmatika,suku ke 4 : 22 dan suku ke 7: 34,jumlah 15 suku pertama pada barisan tersebut adalah.. 2. Pada suatu barisan geometri suku ke2:

1.) U4 = a + 3b = 22
U7 = a + 6b = 34
eliminasi > -3b = -12
b = 4

a + 3b = 22
a + 3(4) = 22
a + 12 = 22
a = 10

Sn = n/2 ( 2a + (n - 1) b )
= 15/2 ( 2(10) + ( 15-1) 4)
= 15/2 ( 20 + 56 )
= 570

2.) U2 = ar = 3
U4 = ar^3 = 27
eliminasi > r^2 = 9
r^2 = 3^2
r = 3

ar = 3
a(3) = 3
a = 1

Sn = a (r^n - 1)/(r - 1)
S7 = 1 ( 3^7 - 1) / ( 3 - 1 )
S7 = 2186 / 2
S7 = 1093

3.) a = 51
b = 3
Un = 81

Un = a + ( n - 1 )b
81 = 51 + ( n - 1 )3
81 = 51 + 3n - 3
-3n = 51 - 3 - 81
-3n = -33
n = 11

Sn = n/2 ( 2a + ( n - 1 )b)
= 11/2 ( 2(51) + ( 11 - 1 )3)
= 11/2 ( 102 + 30 )
= 726

4.) a = 20
b = 4
Un = 88

Un = a + ( n - 1 )b
88 = 20 + ( n - 1 ) 4
88 = 20 + 4n - 4
-4n = 20 - 4 -88
-4n = -72
n = 18

Sn = n/2 ( 2a + ( n - 1 )b)
= 18/2 ( 2(20) + ( 18 - 1 )4)
= 9 ( 40 + 68 )
= 9 ( 108 )
= 972