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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- Aug 13th 2011, 04:17 PMRK29Another SAT Question
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 - Aug 13th 2011, 04:51 PMSammySRe: SAT question
- Aug 13th 2011, 04:53 PMRK29Re: SAT question
well, i tried each option but non of them give me zero. I used number 4 for a and 2 for b

- Aug 13th 2011, 05:04 PMSammySRe: SAT question
Just picking any two numbers to plug in, virtually never gets the job done --- especially for this problem.

For a#b:Use the definition of a#b, set it to zero & solve for a and/or b.

a-ab = 0

a(1-b) = 0

What are the solutions? - Aug 13th 2011, 06:48 PMRK29Re: SAT question
- Aug 13th 2011, 07:08 PMPlatoRe: Another SAT Question
- Aug 13th 2011, 07:32 PMSammySRe: 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) . - Aug 14th 2011, 07:37 AMHallsofIvyRe: 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".