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

sn = n/2 ( 2a + (n-1) b )
42 = n/2 ( 30 + (n-1) -3)
84 = n ( 33 - 3n)
84 = 33n - 3n²
3n² - 33n + 48 = 0
n² - 11n + 28 = 0
( n - 4 )( n - 7 ) = 0
n = 4 atau n = 7

Sn=n/2(2a+(n-1)b)
42=n/2(2×15+(n-1)-3)
84=n(30-3n+3)
84=n(-3n+33)
-3n²+33n-84=0
3n²-33n+84=0
(3n-12)(n-7)
n=4 atau n=7