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1, -6, -9, 0, 45, 198, ?

Category:	Series
Submitted By:	shenqiang
Fun:	(1.69)
Difficulty:	(3.03)

What's the next number in this series?

1, -6, -9, 0, 45, 198, ?

Answer:

675.

-6=1*3+(-9)
-9=-6*3+9
0=-9*3+27
45=0*3+45
198=45*3+63

-9, 9, 27, 45, 63 form an arithmetic sequence with common difference 18.

and therefore, the next number is
675=198*3+81.

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