dalam suatu deret aritmatika suku ke-4 = b5 suku ke 8 = 125, hitunglah a. bedanya b. suku pertamanya c. suku ke-15 d. jumlah 20 suku

a. U4 = a+3b = 105
U8 = a+7b = 125
Eliminasi = U4 = a+3b = 105
U8 = a+7b = 125
____________ _
-5b = -20
b = -20 : (-5) = 4
b. Mencari a atau U1 = a+3b = 105
= a+3.4 = 105
= a+12 = 105
= a = 105 - 12 = 93
c. U15 = a+14b = 93+(14.4) = 93+56 = 149
d. S20 = n/2(2a+(n-1)b)
= 20/2(2.93+(20-1)4)
= 10(186+19.4)
= 10(186+76)
= 10.262 = 2620