# equation

• May 27th 2008, 08:35 AM
tomjkear
equation
Hi,

I have an exam in the next couple of days that I am really not looking forward to.

Can anyone explain to me how the 6.8CR is derived in the following sum?
I see how we get to 13, but 6.8CR is a mystery to me.

http://img388.imageshack.us/img388/1...eenshotgb0.png

Thanks,
TK
• May 27th 2008, 08:56 AM
earboth
Quote:

Originally Posted by tomjkear
Hi,

I have an exam in the next couple of days that I am really not looking forward to.

Can anyone explain to me how the 6.8CR is derived in the following sum?
I see how we get to 13, but 6.8CR is a mystery to me.

http://img388.imageshack.us/img388/1...eenshotgb0.png

Thanks,
TK

Expand the brackets (I'm goin to use $x = C_R$. It's easier to type:

$(2 - 0.8x)^2+(-3+0.6x)^2=4-3.2x+0.64x^2 + 9 - 3.6x + 0.36x^2$

Collect like terms and you'll get the given result.

Probably you missed that

$(2-0.8x)^2 = (2-0.8x)(2-0.8x)$
• May 27th 2008, 09:07 AM
tomjkear
Quote:

Originally Posted by earboth
Expand the brackets (I'm goin to use $x = C_R$. It's easier to type:

$(2 - 0.8x)^2+(-3+0.6x)^2=4-3.2x+0.64x^2 + 9 - 3.6x + 0.36x^2$

Collect like terms and you'll get the given result.

Probably you missed that

$(2-0.8x)^2 = (2-0.8x)(2-0.8x)$

Thanks, but I still don't understand :(
• May 27th 2008, 09:21 AM
topsquark
Quote:

Originally Posted by tomjkear
Thanks, but I still don't understand :(

FOIL it out. For example:
$(2 - 0.8x)^2 = (2 - 0.8x)(2 - 0.8x) = 2 \cdot 2 + 2 \cdot -0.8x + -0.8x \cdot 2 + -0.8x \cdot -0.8x$

$= 4 - 1.6x - 1.6x + 0.64x^2 = 4 - 3.2x + 0.64x^2$

Try that on your other term.

-Dan
• May 27th 2008, 11:10 AM
earboth
Quote:

Originally Posted by tomjkear
Thanks, but I still don't understand :(

I would like to help you but you have to describe exactly what you didn't understand.
• May 27th 2008, 11:27 AM
tomjkear
Quote:

Originally Posted by earboth
Expand the brackets (I'm goin to use $x = C_R$. It's easier to type:

$(2 - 0.8x)^2+(-3+0.6x)^2=4-3.2x+0.64x^2 + 9 - 3.6x + 0.36x^2$

Collect like terms and you'll get the given result.

Probably you missed that

$(2-0.8x)^2 = (2-0.8x)(2-0.8x)$

Quote:

Originally Posted by earboth
I would like to help you but you have to describe exactly what you didn't understand.

It's OK, both your post and the post from the person above have clarified, thanks very much :)