Tricky DS question.

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madhavi: Senior | Next Rank: 100 Posts; Posts: 57; Joined: Tue Jun 26, 2007 11:33 pm; Thanked: 4 times

by madhavi » Sat Nov 17, 2007 8:27 am

If a2 + b2 =13, what is the value of ab?

(1) (a + b)2 = 25.

(2) a(ab3) = 108.

Can't we solve (13-b2)b3=108? And will it not be Choice D?
The answer given is A.

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Source: Beat The GMAT — Data Sufficiency | Sat Nov 17, 2007 8:27 am

ri2007: Master | Next Rank: 500 Posts; Posts: 258; Joined: Mon Aug 27, 2007 12:43 pm; Thanked: 15 times

by ri2007 » Sat Nov 17, 2007 9:04 am

Its so easy to forget small rules of number properties.

any way statement 2 is

a(ab^3) = 108

so a^2 b^3 = 2^2 * 3^3

So b = 3 (OK)
but a = + or - 2

the statement in the stem will be satisfied with either values of a since it includes a a^2 but the question needs u to identify the value of a which can be negative or positive and this thus will give diff values for 2ab

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