-2,1,4……….
tentukan suku ke 10 dan jumlah suku ke 10
2.jika U4=6 dan U8=22
tentukan U1,b,danU10
1.jika barisan bilangan
Jawaban:
1. pola = -2, 1, 4, ..
rumus un = a + ( n – 1 ) b
a = -2
• cari b
b = u2 – u1
b = 1 – (-2)
b = 3
• suku ke 10
un = a + ( n – 1 ) b
u10 = -2 + ( 10 – 1 ) 3
u10 = -2 + 9 x 3
u10 = 25
• jumlah 10 suku pertama
sn = n/2 ( a + un )
s10 = 10/2 ( -2 + 25 )
s10 = 5 x 23
s10 = 115
2 u4 = 6
u8 = 22
rumus un = a + ( n – 1 ) b
• buat persamaan
un = a + ( n – 1 ) b
u4 = a + ( 4 – 1 ) b
6 = a + 3b
un = a + ( n – 1 ) b
u8 = a + ( 8 – 1 ) b
22 = a + 7b
• cari beda
a + 3b = 6
a + 7b = 22
__________ _
-4b = -16
b = 4
• cari u1
a + 3b = 6
a + 12 = 6
a = -6
• nilai u10
un = a + ( n – 1 ) b
u10 = -6 + ( 10 – 1 ) 4
u10 = -6 + 9 x 4
u10 = -6 + 36