Diketahui suatu deret aritmetika: 1 + 4 + 7 + 10 + ... + x = 852. Nilai x adalah

jawab
a=1
b = 3

un = a + (n-1)b
x = 1 + 3n - 3
x= 3n - 2

Sn = n/2 (a + un)
852 = n/2 (1+ 3n - 2)
1704 = n(3n -1)
3n²- n - 1704= 0
(3n+71)(n - 24) =0
n = 24

x = 3n  -2
x = 3(24) - 2
x = 72 - 2
x = 70

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