Jika 2+5+8+...+x =187, maka 3x+7 =

Rumua deret aritmatika :
Un = a + (n - 1)b
x = 2 + (n - 1)3
x = 2 + 3n - 3
x = 3n - 1
3n = x + 1
n = (x + 1)/3


Sn = n/2 × (U1 + Un)
187 = n/2 × (2 + x)
187 × 2 = n × (2 + x)
374 = (x + 1)/3 × (2 + x)
374 × 3 = (x + 1) (2 + x)
1122 = 2x + x^2 + 2 + x
1122 = x^2 + 3x + 2
x^2 + 3x - 1120 = 0
(x + 35) (x - 32) = 0

Maka x = 32


Jadi,
3x + 7 = 3 × 32 + 7 = 96 + 7 = 103