for all numbers x and y, let the operation # be defined as x#y=x-xy. If a and b are positive integers, whuich can be equal to zero?

1) a#b

2) (a+b)#b

3) A # (a+b)

1 only

2 only

3 only

1 and 3

1 and 2

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- August 13th 2011, 04:17 PM #1

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- August 13th 2011, 04:51 PM #2

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- August 13th 2011, 04:53 PM #3

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- August 13th 2011, 05:04 PM #4

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- August 13th 2011, 06:48 PM #5

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- August 13th 2011, 07:08 PM #6

- August 13th 2011, 07:32 PM #7

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## Re: SAT question

Those are two independent solutions. In other words: Either a=0, OR b = 1, will ensure that a#b = 0. As

**Plato**points out, 0 is not positive, so that leaves you with b=1. Of course (check for yourself) if b = 1, then a can be any (positive for this problem) number.

So you know that 1)**can**be equal to zero.

Now try 2) and 3) .

- August 14th 2011, 07:37 AM #8

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## Re: SAT question

x#y= x- xy= x(1- y). In order to be 0, either x= 0 or y= 1.

1) a#b. x= a, b= y. a cannot be 0 but b can be 1.

2)(a+ b)#b. x= a+ b, y= b.

3)a#(a+ b). x= a, y= a+ b. Remember that a and b must be**positive**integers.

This looks to me exactly like your previous "SAT question".