tentukan unsur-unsur a=15,b=-3,sn=42,n=...

Semoga bermanfaat yaa

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

Sn = 42
n/2 . (2a + (n - 1) . b) = 42
n/2 . (2 . 15 + (n - 1) . (-3)) = 42
n (30 - 3n + 3) = 42 . 2
-3n² + 33n - 84 = 0
n² - 11n + 28 = 0 
(n - 7) (n - 4) = 0
n - 7 = 0
n = 7

n - 4 = 0
n = 4