Jawabanmu

2014-05-29T08:09:17+07:00
Sn = (n/2)(2a + (n-1)b)
20 = (n/2)(2(8) + (n-1)(-2))
40 = n(16 - 2n + 2)
40 = - 2n² + 18n
2n² - 18n + 40 = 0
n² - 9n + 20 = 0
(n-4)(n-5) = 0
Jadi, n = 4 atau n = 5