Question

Tentukan nilai n jika diketahui Sn=42, a=15, dan b=-3

Answer 1

Sn = n/2 ( 2a + (n-1) b)
42 = n/2 (30 + (-3n) +3 )
84 = 33n - 3n²
3n²-33n+84 = 0
n²-11n+28 = 0
(n-7)(n-4)
n=7 atau n=4

Answer 2

N/ (2a+(n-1)b)=42
n/2(30-3n+3)=42
n/2(33-3n)=42
33n-3/n=84

-3+33n-84=0
-+11n-28=0
(-n+4)(n-7)
n=4 V n=7