Sequence

Danger Phoenix · 11 Dec 2008 at 21:39

2, 3, 7, 17, 39, ??

a) 58
b) 85
c) 65
d) 75

Also, how do you work out the answer?

shifty_uk · 11 Dec 2008 at 21:41

1,234!

Cosimo · 11 Dec 2008 at 21:45

Colonel_Acid · 11 Dec 2008 at 21:45

80087355

Danger Phoenix · 11 Dec 2008 at 21:47

How about one of the given answers?

Jono8 · 11 Dec 2008 at 21:47

85

I think

the differences are 1,4,10,22,46.

get this by multiplying the difference before by 2 and adding 2.

Sinque · 11 Dec 2008 at 21:51

I just got the same answer, I think.

geuben · 11 Dec 2008 at 21:51

I also say 85. The difference of the differences doubles every time.

Dharok · 11 Dec 2008 at 21:56

Not sure what the answer is but I know for sure this is a part of the methodology behind it all.

Could it be...

?!

David91 · 11 Dec 2008 at 22:24

5318008!?

spirit · 11 Dec 2008 at 22:25

Haircut · 11 Dec 2008 at 22:38

2, 3, 7, 17, 39, ??

2 -> 3 is multiplied by 2, subtract 1
3 -> 7 is multiplied by 2, add 1
7 -> 17 is multiplied by 2, add 3
17 -> 39 is multiplied by 2, add 5
so next one will be multiplied by 2, add 7 which is 85 as people have pointed out.

Jono8 · 11 Dec 2008 at 22:43

Haircut said:
2, 3, 7, 17, 39, ??

2 -> 3 is multiplied by 2, subtract 1
3 -> 7 is multiplied by 2, add 1
7 -> 17 is multiplied by 2, add 3
17 -> 39 is multiplied by 2, add 5
so next one will be multiplied by 2, add 7 which is 85 as people have pointed out.

i dont think thats the correct explanation though as the first one doesnt fit.

geuben · 11 Dec 2008 at 22:44

Jono8 said:
i dont think thats the correct explanation though as the first one doesnt fit.

It does if you change subtract 1 to add -1

Jono8 · 11 Dec 2008 at 22:48

geuben said:
It does if you change subtract 1 to add -1

......thats not how these things generally work.

geuben · 11 Dec 2008 at 22:51

Jono8 said:
......thats not how these things generally work.

....and why not?

you add 2 more than the added to the previous number starting from -1

Haircut · 11 Dec 2008 at 22:57

Jono8 said:
i dont think thats the correct explanation though as the first one doesnt fit.

Of course it fits.

Xn+1 = 2 * Xn + 2(n-0.5) where X0 = 2

if you want to define it like that instead?

pingwing · 11 Dec 2008 at 23:33

85 easy

just need some decent drawing skills to show you the (simple) alevel maths

Jono8 · 12 Dec 2008 at 00:03

Haircut said:
Of course it fits.

Xn+1 = 2 * Xn + 2(n-0.5) where X0 = 2

if you want to define it like that instead?

Yeh that makes more sense.

Jono8 · 12 Dec 2008 at 00:04

geuben said:
....and why not?

you add 2 more than the added to the previous number starting from -1

i agree. it was the 'subtract' 1 that sounded stupid in my head lol