Question

Tentukan banyak suku pada deret aritmatika 2+5+8..=55

Answer 1

Sn = n/2 ( 2a + (n-1)b )
55 = n/2 ( 2(2) + bn - b )
55 = n/2 ( 4 + 3n - 3 )
55 = n/2 ( 3n + 1 )
55 = (3n^2 + n) / 2
3n^2 + n = 110
3n^2 + n - 110 = 0

salah soal kayaknya nih