Question

Dik: barisan aritmetika dengan U3= 9 dan U8= 4
tentukan: a. suku pertama dan beda:
b. rumus Un:
c. U1.001
d. S1.001

Answer 1

U3 = a + 2b = 9
U8 = a + 7b = 4
-----------------------(-)
-5b = 5
b = -1
U3 = a + 2b
9 = a + 2(-1)
9 = a - 2
a = 2 + 9 = 11
a) Un = a + (n - 1) b
= 11 + (n - 1) (-1)
= 11 - n + 1
= 12 - n
b) U1001 = 12 - 1001
= -989
c) S1001 = 1001/2 (11 - 989)
= 1001/2 × (-978)
= - 489.489