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1) Un = ar^(n - 1)
U3 = ar^2 = 36
U5 = ar^4 = 81
U5/U3 = (ar^4)/(ar^2) = 81/36
r^2 = 9/4
r = √(9/4)
r = ± 3/2
2) U2 + U1 = 26
a + b + a = 26
2a + b = 26 ... (1)
U3 + U5 = 22
a + 2b + a + 4b = 22
2a + 6b = 22 ... (2)
Kurangkan (2) dengan (1),
5b = -4
b = -4/5
Substitusi, dapet a = 67/5
Un = a + (n - 1)b
= 67/5 + (n - 1)b
U6 = 67/5 + 5(-4/5)
= 67/5 - 20/5
= 47/5