YAMathT: How does that (a+b) * (a-b) thingie work? *UPDATE*

[ndee]

So is (a+b) * (a-b) = a^2 - b^2 ?




I have this problem: For which value can p have two solutions: 2x^2 + px + 32 = 0.

The solution is 16 so it's not like it's homework, I just wanna know the way.

I came to the conclusion that you need something like (2x + 4) (x + 8) = 0 but then, I'm stuck.

Thanks again.

 

[MrChad]

First, Outer, Inner, Last

(a+b)(a-b) = a^2 + ab - ab - b^2 = a^2 - b^2

 

[Demon-Xanth]

a*a-a*b+b*a-b*b

 

[ElFenix]

and to solve it is x = (-b +/- sqrt(b^2-4ac))/2a

 

[BlueWeasel]

[Edit] MrChad beat me to it.

I was taught the FOIL method...First, Outside, Inside, Last

(x-5)*(x+3)

First....x*x = x^2
Outside....x*(+3) = 3x
Inside....(-5)*x = -5x
Last....(-5)*(+3) = -15

Now just add together the values....x^2 + 3x - 5x - 15 = x^2 - 2x - 15

 

[ndee]

ok, and (a-b) * (a-b) = (a-b)^2 right? And (a+b) * (a+b) = (a+b) ^ 2

But I remember there's also another thingie, where the end result is 2ab + b + a^2 or something.

I'm going to school again after 2 years of working and I'm not really sure anymore, thanks guys.

 

[ndee]

Originally posted by: BlueWeasel
[Edit] MrChad beat me to it.

I was taught the FOIL method...First, Outside, Inside, Last

(x-5)*(x+3)

First....x*x = x^2
Outside....x*(+3) = 3x
Inside....(-5)*x = -5x
Last....(-5)*(+3) = -15

Now just add together the values....x^2 + 3x - 5x - 15 = x^2 - 2x - 15

That was what I'm talking about, just gonna read that up, thanks!

 

[Gibson486]

Originally posted by: ndee
ok, and (a-b) * (a-b) = (a-b)^2 right? And (a+b) * (a+b) = (a+b) ^ 2

But I remember there's also another thingie, where the end result is 2ab + b + a^2 or something.

I'm going to school again after 2 years of working and I'm not really sure anymore, thanks guys.

no
(a+b)^2 = a^2 +2ab + b^2.

 

[ElFenix]

Originally posted by: ndee
ok, and (a-b) * (a-b) = (a-b)^2 right? And (a+b) * (a+b) = (a+b) ^ 2

But I remember there's also another thingie, where the end result is 2ab + b + a^2 or something.

I'm going to school again after 2 years of working and I'm not really sure anymore, thanks guys.

normally you have (x + a)(x + b) = x^2 + (a + b)x + ab = 0

 

[ndee]

Originally posted by: Gibson486

Originally posted by: ndee
ok, and (a-b) * (a-b) = (a-b)^2 right? And (a+b) * (a+b) = (a+b) ^ 2

But I remember there's also another thingie, where the end result is 2ab + b + a^2 or something.

I'm going to school again after 2 years of working and I'm not really sure anymore, thanks guys.

no
(a+b)^2 = a^2 +2ab + b^2.

ah ok.

 

[BlueWeasel]

FOIL Method

 

[ndee]

Originally posted by: BlueWeasel
FOIL Method

great page! Thanks, gonna check that site up!

 

[ndee]

Bump.