# Phys Chem blues

• June 4th 2008, 10:20 PM
chem_student
Phys Chem blues
Hi all,

Im having trouble solving for x in this equation.

x
4.95 = ______________
(0.7 - x)(0.7 - x)

I have tried re-teaching myself the quadratic equation, completing the square etc.. but dont know how to apply it here.

I know the answer is 0.412 or -1.19.

I will need this skill for an upcoming exam without a doubt.

• June 4th 2008, 10:41 PM
Chris L T521
Quote:

Originally Posted by chem_student
Hi all,

Im having trouble solving for x in this equation.

x
4.95 = ______________
(0.7 - x)(0.7 - x)

I have tried re-teaching myself the quadratic equation, completing the square etc.. but dont know how to apply it here.

I know the answer is 0.412 or -1.19.

I will need this skill for an upcoming exam without a doubt.

Don't worry! I'm here to help! :D

$4.95=\frac{x}{(0.7-x)^2}$

Multiplying both sides by the denominator, we have the following:

$4.95(0.7-x)^2=x$

Expanding the $(0.7-x)^2$ term, we get the following:

$4.95(.49-1.4x+x^2)=x$

Distribute the 4.95 to get:

$4.59x^2-6.426x+2.2491=x$

Subtract x from both sides to get a quadratic:

$4.59x^2-7.426x+2.2491=0$

Now solve for x using the quadratic formula:

$x=\frac{-(-7.426)\pm\sqrt{(-7.426)^2-4(4.59)(2.2491)}}{2(4.59)}$

This gives us two solutions: $\color{red}\boxed{x=.404}$ or $\color{red}\boxed{x=1.214}$.

If you have any questions feel free to ask! :D
• June 5th 2008, 04:40 AM
earboth
Quote:

Originally Posted by Chris L T521
Don't worry! I'm here to help! :D

$4.95=\frac{x}{(0.7-x)^2}$

Multiplying both sides by the denominator, we have the following:

$4.95(0.7-x)^2=x$

Expanding the $(0.7-x)^2$ term, we get the following:

$4.95(.49-1.4x+x^2)=x$

Distribute the 4.95 to get:

$\underbrace{4.59}_{wrong\ number}x^2-6.426x+2.2491=x$ *****

Subtract x from both sides to get a quadratic:

$4.59x^2-7.426x+2.2491=0$

Now solve for x using the quadratic formula:

$x=\frac{-(-7.426)\pm\sqrt{(-7.426)^2-4(4.59)(2.2491)}}{2(4.59)}$

This gives us two solutions: $\color{red}\boxed{x=.404}$ or $\color{red}\boxed{x=1.214}$.

If you have any questions feel free to ask! :D

Unfortunately you have changed the order of digits when calculating the solution.

I've got x = 0.41163 or x = 1.19039
The second solution is positive too :confused: