Diketahui deret aritmatika (x + y), (2x + 3y), (6x-3y), … jika beda antar suku b = 4 maka suku ke-25 adalah…​

U2 – U1 = U3 – U2
2x + 3y – x – y = 6x – 3y – 2x – 3y
x + 2y = 4x – 6y
– 3x + 8y = 0

U2 – U1 = b
2x + 3y – x – y = 4
x + 2y = 4

– 3x + 8y = 0
3x + 6y = 12

14y = 12
y = 6/7

x + 2y = 4
x + 2(6/7) = 4
x + 12/7 = 4
x = 4 – 12/7
x = 16/7

a = x + y
a = 16/7 + 6/7
a = 22/7

Un = a + (n – 1)b
U25 = 22/7 + (25 – 1)4
U25 = 22/7 + 96
U25 = 99 1/7
U25 = 694/7