GR9768 Q 51

Forum for the GRE subject test in mathematics.
mathQ
Posts: 41
Joined: Thu Mar 25, 2010 12:14 am

GR9768 Q 51

Postby mathQ » Tue Apr 06, 2010 1:10 pm

let D be the region in the xy plane in which the series

Summation (x+2y)^k / k converges.

then the interior of D is

A) an open disk
B) the open region bounded by an ellipse
C) the open region bounded by a quadrilateral
D) the open region bounded by 2 parallel lines
E) an open half plane

speedychaos4
Posts: 47
Joined: Mon Mar 22, 2010 2:42 am

Re: GR9768 Q 51

Postby speedychaos4 » Tue Apr 06, 2010 1:28 pm

is it D?

mathQ
Posts: 41
Joined: Thu Mar 25, 2010 12:14 am

Re: GR9768 Q 51

Postby mathQ » Tue Apr 06, 2010 1:31 pm

speedychaos4 wrote:is it D?



Yes, can you explain ? Thanks

speedychaos4
Posts: 47
Joined: Mon Mar 22, 2010 2:42 am

Re: GR9768 Q 51

Postby speedychaos4 » Tue Apr 06, 2010 1:49 pm

Use the ratio test, you can get r=|x+2y|, in order to ensure convergence, you want r<1, and therefore you have the ans.

mathQ
Posts: 41
Joined: Thu Mar 25, 2010 12:14 am

Re: GR9768 Q 51

Postby mathQ » Tue Apr 06, 2010 2:02 pm

speedychaos4 wrote:Use the ratio test, you can get r=|x+2y|, in order to ensure convergence, you want r<1, and therefore you have the ans.


Great.

Thanks Speedy




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