A number theory problem by A Former Brilliant Member

Number Theory Level 2

What is the next number in this series?

4, 12, 84, 3612....


The answer is 6526884.

A Former Brilliant Member by A Former Brilliant Member

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2 solutions

Gowtham Amirthya
Apr 8, 2014

A simpler way is take the ratios.... r1= 12/4=3 r2 = 84/12 = 7 r3 = 3612/84 = 43 This is known as Sylvester's sequence ( http://en.wikipedia.org/wiki/Sylvester%27s_sequence ) 2, 3, 7, 43, 1807, 3263443, 10650056950807, ..... r4= 1807 So required term is 1807 x 3612 = 6526884

1 Helpful 0 Interesting 0 Brilliant 0 Confused
Bibin Viyyath
Feb 28, 2014

n *(n/2 +1)

1 Helpful 0 Interesting 0 Brilliant 0 Confused

To get from 4 to 12, you use the following formula. 12 = (4 squared)/2 + 4. (X^2)/2 + X.

Kařtīk Gupta - 7 years, 2 months ago

That's hard to figure it out !!! i'm soo mad

Mehdi Asta - 7 years, 3 months ago

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