Q56 on GRE practicing to take the mathematics test

Forum for the GRE subject test in mathematics.
huayualice
Posts: 15
Joined: Thu Sep 11, 2014 7:19 am

Q56 on GRE practicing to take the mathematics test

Postby huayualice » Thu Sep 11, 2014 8:54 pm

56. If k is a real number and f(x) = sin(1/x) for x not equal to 0, and f(x)=k for x=0
and if the graph of f is not a connected subset of the plane, then the value of k

(A) could be -1
(B) must be 0
(C) must be 1
(D) could be less than 1 and greater than -1
(E) must be less than -1 or greater than 1

Answer is (E)

Can anyone help on this? I can make sense of (E), but how do you prove it?

DDswife
Posts: 60
Joined: Thu Aug 14, 2014 5:29 pm

Re: Q56 on GRE practicing to take the mathematics test

Postby DDswife » Thu Sep 11, 2014 11:02 pm

Which number is this GRE exam?

DDswife
Posts: 60
Joined: Thu Aug 14, 2014 5:29 pm

Re: Q56 on GRE practicing to take the mathematics test

Postby DDswife » Thu Sep 11, 2014 11:10 pm

You don't want a connected subset of the plane. Hence, k cannot be in [-1,1]. As you can see, the function and you will see that it gets 'crazy' about 0

http://www.webalice.it/extrabyte/sin1_x.jpg
Last edited by DDswife on Fri Sep 12, 2014 3:41 pm, edited 2 times in total.

thomasmgill
Posts: 2
Joined: Thu Sep 11, 2014 9:36 pm

Re: Q56 on GRE practicing to take the mathematics test

Postby thomasmgill » Fri Sep 12, 2014 9:48 am

Since the range of f is a subset of R, to determine if it is a connected subspace, we must find if the range of f is an interval.

Since -1<=sin(1/x)<=1 for all x not zero, and it is continuous for x not zero, then the range of f is {[-1,1]Uf(0)}.

Clearly this is an interval if f(0) is in [-1,1], and not an interval is f(0) is not in [-1,1]. Thus the answer is E, that f(0) must be less than -1 or greater than 1.

huayualice
Posts: 15
Joined: Thu Sep 11, 2014 7:19 am

Re: Q56 on GRE practicing to take the mathematics test

Postby huayualice » Sat Sep 13, 2014 11:08 pm

Thanks! I was overthinking, now it seems so obvious...




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