Question

Diketahui barisan aritmatika dengan U5 = 8 dan U9 = 20 . Suku ke-10 adalah . . .
a. -31
b. -33
c. 23
d. 31

Answer 1

Un = a + (n - 1)b

U5 = a + 4b = 8
U9 = a + 8b = 20
U10 ?

U9 - U5 = 

a + 8b = 20
a + 4b = 8
------------------- -
4b = 12
b = 3

input b = 3
a + 4b = 8
a + 12 = 8
a = -4

U10 = a + 9b
input a = -4; b = 3;
U10 = (-4 ) + 9(3)
U10 = -4 + 27 = 23 (c)

Answer 2

U5 = 8
U9=20
maka cari beda = 20-8 = 12    = 3  
                           9-5      4
maka a
U5=a+4b
8 = a+ 4.3
8 = a+12
a=8-12 = -4

maka a=-4, b=3

U10=a+9b
U10=-4 + (9.3)
      = -4 +27 = 23 (c)