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# Factoring question

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Intern
Joined: 11 Oct 2006
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11 Sep 2007, 08:24
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Factors are my weakness, so if this is elementary please forgive me...

Question:

A^2 + B^2 + 2AB = 9

What is A+B?

The steps in explanation goes as follows: "Factor the expression A^2 + B^2 + 2AB. Then Unsquare both sides, taking both the positive and negative roots into account"

Step 1. (A+B)^2 = 9

Answer: 2. A+B = 3 or A +B = -3

My question is what happened to the + 2AB in Step 1?

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Director
Joined: 17 Sep 2005
Posts: 901

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11 Sep 2007, 09:04
boggin wrote:
Factors are my weakness, so if this is elementary please forgive me...

Question:

A^2 + B^2 + 2AB = 9

What is A+B?

The steps in explanation goes as follows: "Factor the expression A^2 + B^2 + 2AB. Then Unsquare both sides, taking both the positive and negative roots into account"

Step 1. (A+B)^2 = 9

Answer: 2. A+B = 3 or A +B = -3

My question is what happened to the + 2AB in Step 1?

Yes, it's a basic question.

We have a formula:

(A+B)^2 = A^2 + B^2 + 2AB

Reverse is also true..i.e:

A^2 + B^2 + 2AB = (A+B)^2

A^2 + B^2 + 2AB = 9
=> (A+B)^2 = 9 (since A^2 + B^2 + 2AB = (A+B)^2)
=> (A+B) = Sqrt(9)
=> (A+B) = +3 or -3

Hope it's clear now.

- Brajesh

Last edited by b14kumar on 11 Sep 2007, 09:41, edited 1 time in total.

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Intern
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Posts: 22

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11 Sep 2007, 09:30
Thanks for the quick reply, guess Its obvious I need to memorize those formulas

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Thanks   [#permalink] 11 Sep 2007, 09:30
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