23, 26, 13, 18, 9, 16, 8, 17, x, yTentukan nilai (x+y).

## Pembahasan

Pola deret: Dua barisan selang-seling.
- Barisan A (posisi ganjil): 23, 13, 9, 8, x
- Beda: −10, −4, −1, 0 → selisih beda: +6, +3, +1 (berkurang setengah kali)
- x = 8 − 0 = 8
- Barisan B (posisi genap): 26, 18, 16, 17, y
- Beda: −8, −2, +1, +3
- y = 17 + 3 = 20

x + y = 8 + 20 = 28

Jawaban: A (28)