Question

Diket : 1 + 7/2 + 6 + .... Sn = 208
Tentukan n dan Un

Answer 1

1 + 7/2 + 6 + ....
beda = b = 7/2 - 1 = 5/2
Sn = n/2 (2a + (n - 1)b)
208 = n/2 (2.1 + (n - 1).5/2)
208 = n/2 (2 + 5/2 n - 5/2)
208 = n/2 (5/2 n - 1/2)
208 = 5/4 n² - 1/4 n
832 = 5n² - n
5n² - n - 832 = 0
(5n + 64)(n - 13) = 0
n = -64/5 atau n = 13 
ambil n = 13
Un = a + (n - 1)b
U13 = 1 + 12(5/2) = 1 + 30 = 31