Question

Tolong bantu jawab... contoh 8. gimana caranya nih ?

Answer 1

Suku ke-n : Un = a + (n - 1)b
Jumlah n suku : Sn = n/2 {2a + (n - 1)b}

Dari soal,
U2 + U6 = 42
a + b + a + 5b = 42
2a + 6b = 42 (1)

U8 + U16 = 84
a + 7b + a + 15b = 84
2a + 22b = 84 (2)

Eliminasi (1) dan (2) :
2a + 6b = 42
2a + 22b = 84
---------------------- -
16b = 42
b = 2,625

2a + 6b = 42
2a + 6 (2,625) = 42
2a + 15,75 = 42
2a = 26,25
a = 13,125

Jumlah 5 suku pertama :
Sn = n/2 {2a + (n - 1)b}
= 5/2 {26,25 + 4 . 2,625}
= 5/2 . 36,75
= 91,875