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 Post subject: GMAT SymbolsPosted: Wed Oct 05, 2011 3:45 pm

Joined: Sun May 30, 2010 3:15 am
Posts: 424
If a#b = (ab)(a + b) then what is the value of a#b + b#a?
A. -2ab
B. 0
C. ab
D. a² + b²
E. 2ab

(B) Note that when we calculate b#a we must switch a with b in the expression for a#b.

a#bb#a can be calculated in several ways. The fastest way is to see the formula for difference of squares. a#b = (ab)(a + b) = a² – b². Therefore b#a = b² – a². So a#bb#a = a² – b² – a² + b² = 0. The answer is (B).
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What does the # symbol mean in these types of questions?

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 Post subject: Re: math (test 3, qt. 11): symbolsPosted: Wed Oct 05, 2011 3:55 pm

Joined: Sun May 30, 2010 2:23 am
Posts: 498
Quote:
What does the # symbol mean in these types of questions?
Such symbols as #, @, etc., which are not used in the school math, define a new mathematical operation.

In school we learn basic mathematical operations, such as addition (+), subtraction (–), multiplication (×), etc.
Similarly, the # sign defines a new mathematical operation. a#b IS (a – b)(a + b).

4#3 = (4 – 3)(4 + 3) = 7
1#2 = (1 – 2)(1 + 2) = -3
....

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