diketahui suku ke-3 = 15,suku ke-9 = 18 tentukan 2 suku awal b.bedanya c. N12 D. S10 Tolong..!!

jawab

u3 = 15
u9 =  18


u9 - u3 = 18 - 15
(9-3)b = 3
6b = 3
b = 3/6 = 1/2

u3 = 15
a + 2b = 15
a + 2(1/2) = 15
a + 1 = 15
a = 14

a) dua suku awal 
a = 14
u2 = a + b = 14 + 1/2 = 14 ¹/₂ = 14, 5

b)  beda = 1/2 = 0,5

c). U12 = a + 11b  =  14 + 11(1/2) = 14 + 5,5 = 19,5
 
d) Sn = n/2  (2a + (n-1)b)
s10 = 10/2 ( 2(14) - 9(1/2)
s10 =5 (28 - 4.5)
s10= 5(23,5)
s10 = 117,5.