## GR0568 problem 25 help!

Forum for the GRE subject test in mathematics.
qwertyGuy
Posts: 4
Joined: Thu Aug 20, 2009 1:07 am

### GR0568 problem 25 help!

I am trying this problem -
{a_n} is a recursively defined sequence with a_{n+1} = (n+2)/n * a_n . The problem asks for a_30. The answer I get is 32!/(30!*3!) . I checked the asnwer on matlab and it is correct. But it is not one of the choices given. Instead it says the correct answer is choice A which is (15)(31) - my question is what does (15)(31) mean, I have never seen this symbol before.

Any help would be appreciated. Thanks!

diogenes
Posts: 73
Joined: Sun Aug 31, 2008 9:31 pm

### Re: GR0568 problem 25 help!

Note a_30 = a_29+1.

Then expand and cancel like terms, as you did, and you will get (31)(30)/2 = (31)(15).

qwertyGuy
Posts: 4
Joined: Thu Aug 20, 2009 1:07 am

### Re: GR0568 problem 25 help!

thanks, diogenes!
but my problem is that i don't know what (31) means! this is probably a really stupid question, but i haven't done a lot of math
could you tell me what that means ...it would be much appreciated! (i tried googling but it didn't help)

qwertyGuy
Posts: 4
Joined: Thu Aug 20, 2009 1:07 am

### Re: GR0568 problem 25 help!

it just means 15*31 aaaaaaaaaaaaaaaahhhhhhhhhhhhhhh
sorry, it was stupid - i just thought it was something profound. thanks