# Subtraction of binomial fractions?

• Dec 7th 2011, 08:10 PM
AlexWilson94
Subtraction of binomial fractions?
I'm SO close to getting the answer, but where do I go from here?

http://oi42.tinypic.com/2505ct3.jpg
• Dec 7th 2011, 08:17 PM
Re: Subtraction of binomial fractions?
you cannot take $(a^2+b^2)=(a+b)^2$ (which you've done in 3rd step) just expand and simplify
• Dec 7th 2011, 08:21 PM
AlexWilson94
Re: Subtraction of binomial fractions?
Are you sure? Why can't I do that? :(

Edit: Either way I get the same answer, which I don't know what to do with..
• Dec 7th 2011, 08:23 PM
pickslides
Re: Subtraction of binomial fractions?
Remember $a^2+b^2 \neq (a+b)^2$

Spoiler:
$\displaystyle x = \sqrt{\frac{10\times 30^2-30\times 10^2}{20}}$

• Dec 7th 2011, 08:26 PM
Re: Subtraction of binomial fractions?
because you, yourself has taken $(x+30)^2=(x^2+60x+30^2)$ at one point (on the fifth line)

so clearly it is not (x^2+30^2)
• Dec 7th 2011, 08:27 PM
AlexWilson94
Re: Subtraction of binomial fractions?
Quote:

Originally Posted by pickslides
Remember $a^2+b^2 \neq (a+b)^2$

Spoiler:
$\displaystyle x = \sqrt{\frac{10\times 30^2-30\times 10^2}{20}}$

Thank you very much, sorry about the attachment!

So.. How did you just do that?
• Dec 7th 2011, 08:28 PM
pickslides
Re: Subtraction of binomial fractions?
Quote:

Originally Posted by AlexWilson94
Are you sure? Why can't I do that? :(

pick a=2 and b=3, try it for both sides, are they the same?
• Dec 7th 2011, 08:29 PM
AlexWilson94
Re: Subtraction of binomial fractions?
Quote:

because you, yourself has taken $(x+30)^2=(x^2+60x+30^2)$ at one point (on the fifth line)

so clearly it is not (x^2+30^2)

Oohh.. My bad on that.. Sorry, thank you!
• Dec 7th 2011, 08:30 PM
AlexWilson94
Re: Subtraction of binomial fractions?
Quote:

Originally Posted by pickslides
pick a=2 and b=3, try it for both sides, are they the same?

It clicks now.. Got it, thanks for clearing that up!
• Dec 7th 2011, 08:30 PM
pickslides
Re: Subtraction of binomial fractions?
Quote:

Originally Posted by AlexWilson94

So.. How did you just do that?

With pen and paper, you have been told where you went wrong, so have another go at it, post your attempts if you need further clarification.

• Dec 7th 2011, 08:43 PM
AlexWilson94
Re: Subtraction of binomial fractions?
So from the 3rd line, I went:

(10x^2 + 9000) - (30x^2 + 1000) = 0
-20x^2 + 6000 = 0
6000 = -20x^2

Then I took the square root of both sides, then divided to square root of 6000 by 20, and wualah. x=3.87 :)

Edit: By the way, how do you get the math symbols? :)
• Dec 7th 2011, 08:52 PM
pickslides
Re: Subtraction of binomial fractions?
Quote:

Originally Posted by AlexWilson94
So from the 3rd line, I went:

(10x^2 + 9000) - (30x^2 + 1000) = 0
-20x^2 + 6000 = 0
6000 = -20x^2

Then I took the square root of both sides, then divided to square root of 6000 by 20, and wualah. x=3.87 :)

This does not look correct.

$6000 = -20x^2 \implies x= \sqrt{\frac{6000}{-20}}\neq 3.87$

Quote:

Originally Posted by AlexWilson94

Edit: By the way, how do you get the math symbols? :)

Put "tex" tags around your equations
• Dec 7th 2011, 09:03 PM
AlexWilson94
Re: Subtraction of binomial fractions?
[/tex]
Quote:

Originally Posted by pickslides
This does not look correct.

$6000 = -20x^2 \implies x= \sqrt{\frac{6000}{-20}}\neq 3.87$