Is this triangle isosceles?

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netigen: Legendary Member; Posts: 631; Joined: Mon Feb 18, 2008 11:57 pmFollowed by: 3 members; Thanked: 29 times

by netigen » Wed May 28, 2008 1:58 am

Is the triangle ABC isosceles?

(A) AB = BC
(B) AC != AB

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Source: Beat The GMAT — Data Sufficiency | Wed May 28, 2008 1:58 am

pranavc: Master | Next Rank: 500 Posts; Posts: 167; Joined: Tue Feb 26, 2008 4:37 amFollowed by: 1 member; Thanked: 5 times

by pranavc » Wed May 28, 2008 3:44 am

My guess is that the answer should be C. The first statement tells you two sides are equal but there is no information about the third side. Hence, at this point, the triangle could be equilateral too. The second statement explicitly states that two of the sides are equal, but does not give information about a third side. Hence, by itself, it cannot provide us with enough information to determine if the triangle is isosceles, but the statements together provide information on all three sides and help us answer the question. Hence. the answer is C. Please let me know if this is incorrect. Thanks.

Pranav

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chidcguy: Master | Next Rank: 500 Posts; Posts: 438; Joined: Mon Feb 12, 2007 9:44 am; Thanked: 26 times

by chidcguy » Wed May 28, 2008 10:39 am

Whew! I should have seen this coming. When i easily deduced that A is the correct answer, I should have thought for a second. Is there a trap?

While I agree with pranavc's explanation, Don't we have enough sufficiency to say that ABC is isosceles,. It can be equilateral or not, but thats not we are asked. we are asked whether based on the info given in A can we say Yes/No.

Stuart/Tatiana??

Thanks

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netigen: Legendary Member; Posts: 631; Joined: Mon Feb 18, 2008 11:57 pmFollowed by: 3 members; Thanked: 29 times

by netigen » Wed May 28, 2008 8:11 pm

This is a good example of a case where we tend to miss the minor points. Whats the definition of an isosceles triangle? Any triangle with two equal sides is isosceles hence A is sufficient.

Note that equilateral triangle is a special case of isosceles triangle.

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chidcguy: Master | Next Rank: 500 Posts; Posts: 438; Joined: Mon Feb 12, 2007 9:44 am; Thanked: 26 times

by chidcguy » Thu May 29, 2008 7:15 am

Thanks for letting us know that A is the answer. I was a bit surprised by pranavc's logic and was worried about if GMAT really goes to such lengths.

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pranavc: Master | Next Rank: 500 Posts; Posts: 167; Joined: Tue Feb 26, 2008 4:37 amFollowed by: 1 member; Thanked: 5 times

by pranavc » Thu May 29, 2008 7:27 am

Thanks for the clarification guys. I can't believe I actually excluded equilateral triangles from isosceles trianges.

Pranav

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