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396 = n/2(2+(n-1)7)
396 x 2 = n(2+7n-7)
792 = n(7n-5)
7n^2 -5n -792 = 0
(7n+72)(n-11) =0
7n = -72 tdk memenuhi
n-11=0
n = 11
jd bntak suku pd deret itu ada 11 suku
b = U2 - U1
= 8 - 1
= 7
Sn = 396
n
Sn = --- {2a + (n - 1)b}
2
n
396 = --- {2(1) + (n - 1)7}
2
n
396 = --- {2 + 7n - 7}
2
n
396 = --- {7n - 5}
2
n(7n - 5)
396 = ------------
2
7n² - 5n
396 = ------------
2
396 x 2 = 7n² - 5n
792 = 7n² - 5n
7n² - 5n - 792 = 0
(7n + 72)(n - 11)
7n + 72 = 0 atau n - 11 = 0
7n = - 72 (TM) n = 11
Pembuktian
n
Sn = --- {2a + (n - 1)b}
2
11
Sn = --- {2(1) + (11 - 1)7}
2
11
Sn = --- {2 + (10)7}
2
11
Sn = --- {2 + 70}
2
11
Sn = --- {72}
2
792
Sn = -----
2
Sn = 396 (Terbukti)