tentukan suku ke15 barisan tsb adalah

U1 = 2x + 1
U2 = x + 2
U3 = 2x - 3
2 × U2 = U1 + U3
2(x + 2) = (2x + 1) + (2x - 3)
2x + 4 = 4x - 2
4 + 2 = 4x - 2x
6 = 2x
x = 6/2 = 3
U1 = a = 2(3) + 1 = 7
U2 = 3 + 2 = 5
U3 = 2(3) - 3 = 3
b = U2 - U1 = 5 - 7 = -2

Un = a + (n -1)b
U15 = 7 + (15-1)(-2)
= 7 - 28
= -21