jika u5 =8 dan u9=20 tentukan 9suku pertama

b = (U9 - U5) / (9 - 5)
b = (20 - 8) / 4
b = 12 / 4
b = 3

U5 = a + 4b
8 = a + 4(3)
a = 8 - 12
a = -4

S9 = 9/2 (2a + 8b)
S9 = 9/2 (2.-4 + 8.3)
S9 = 9/2 (-8 + 24)
S9 = 9/2 (16)
S9 = 9 × 8
S9 = 72

Semoga berguna +_+