Question

Suatu barisan aritmatika diketahui U3 =11 dan U9=23. jumlah 24 suku pertama barisan adalah...

Answer 1

Cari beda


cari suku awal
U3 = a +(3-1)b
U3 = a + 2b
11 = a + ( 2 x 4)
11 = a + 8
a = 11-8
a = 3


Sn = n/2 (2a + (n-1) b)
S24 = 24/2 [(2 x 3) + (24-1)4]
S24 = 12 (6 + ( 23 x 4)
S24 = 12 ( 6 + 92)
S24 = 12 (98)
S24 = 1176

Answer 2

A=7
b=2
U24=a+23b
       = 7+23(2)
       =7+46=53
S24= 24/2(7+53)
      =12×60
      = 720