Erwin Harahap

do Now or Never

Deret bilangan ganjil

Posted by Erwin on September 7, 2015

1 + 3 + 5 + 7 + … + n = 625. Maka nilai n adalah …

Jawab:

Barisan bilangan ganjil.
Un = 2n – 1

Deret jumlah bilangan ganjil:
Sn = (n/2)(2a + (n-1)b)

Sn = 625
a    = 1
b    = 2

maka,
625 = (n/2)(2(1) + (n-1)2)
= n(1 + (n-1))
= n^2

n = akar (625)
n = 25

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