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A math question: og Math Q122 ????

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A math question: og Math Q122 ????

by johnnyyy » Mon Jan 12, 2009 9:41 am
An operation * is defined by the equation a*b = a-b/a+b, for all numbers a and b such that
a doesn't equal -b
a doesn't equal -c

a*c = 0

then c =
-a
-1/a
0
1/a
a

The answer told us to substitute c for b, why ????

Thanks for you help!
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by yasik19 » Mon Jan 12, 2009 9:09 pm
is the answer c?
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by awesomeusername » Tue Jan 13, 2009 1:55 pm
It wants you to substitute the above operation to fit the question at hand. If

a*b=(a-b)/(a+b), then
a*c=(a-c)/(a+c).

We know that "a" is not equal to "-b", which means that "a" and "b" are the same number, but "b" is the negation of a. So if a=5, then b cannot be -5. This is because the equation above will become infinity when the denominator is 0. Therefore we can use this same rule to show that "a" does not equal "-c"

Back to our problem. If we know a*c=(a-c)/(a+c) and a*c = 0, then

(a-c)/(a+c)=0. Since we know that a+c is a non-zero value, that must mean that a-c=0, or a=c. Thus the answer would be E.
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by johnnyyy » Wed Jan 21, 2009 7:59 pm
awesomeusername wrote:It wants you to substitute the above operation to fit the question at hand. If

a*b=(a-b)/(a+b), then
a*c=(a-c)/(a+c).

We know that "a" is not equal to "-b", which means that "a" and "b" are the same number, but "b" is the negation of a. So if a=5, then b cannot be -5. This is because the equation above will become infinity when the denominator is 0. Therefore we can use this same rule to show that "a" does not equal "-c"

Back to our problem. If we know a*c=(a-c)/(a+c) and a*c = 0, then

(a-c)/(a+c)=0. Since we know that a+c is a non-zero value, that must mean that a-c=0, or a=c. Thus the answer would be E.
Well! Suddenly, everything just starts to make sense to me. I thought too deep. It's just a wried question. Anywayz.. thanks for the explanation :D
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