suku ke tiga dan suku kesebelas barisan aritmetika masing masing -6 dan 18 tentukanlah: suku pertama dan bedanya Barisan geometri,diketahui bahwa U2:27 dan U4:

1.)
U3 = -6
U11 = 18

a + (n-1) b = U3
a + (3 - 1) b = -6
a + 2b = -6

a + (11 - 1) b = 18
a + 10b = 18

a + 2b = -6
a + 10b = 18
_________ _
-8b = -24
b = 3

a + 2b = -6
a + 2(3) = -6
a + 6 = -6
a = -12

2.)

a × r^n-1 = U2
a × r^2-1 = 27
a × r = 27

a × r^n-1 = U4
a × r^4-1 = 3
a × r^3 = 3


a × r^3 = 3 = U4
___________
a × r = 27 = U2

r^2 = 3/27
r^2 = 1/9
r = 1/3

a × r = 27
a × 1/3 = 27
a =27/1/3
a = 27 × 3/1
a = 81