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Teachers' Lounge Discussion: Interviewing questions

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From: quantgal <laurie.hailer@gmail.com>
To: Teacher2Teacher Public Discussion
Date: 2009062910:59:55
Subject: Re: mathematics

Assuming your question is sincere, here is a response:

Because you are taking the opposite of the absolute value of the
product two times. The first time (minus times plus) you multiply a
negative times the absolute value product , you will yield a negative
answer. The second negative makes that product positive, when you take
the opposite the second time. 

Example:

-2 x -3 = (-1 X 2) x (-1 x 3)

rearrange:

(2 x 3) x (-1) x (-1)

2 x 3 = +6

Substitute and multiply by the first negative:

(+6) x (-1) = -6

Substitute and multiply by the second negative:

(-6) x (-1) = the opposite of negative 6, which is +6.

Therefore, minus times minus is plus.

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