Hope I'm posting this in the right place =3
Book: Intermediate Algebra: Charles P. Mckeague 9th edition.
Section 8.2 Using the Quadratic formula
The instructions say: "Multiply both sides of the equation by its LCD. Then Solve the resulting equation."

Problem:

"(6/r²-1)-1/2= (1/r+1)"

So naturally I did

{6/(r²-1)} * 2(r²-1)(r+1) - (1/2) * 2(r²-1)(r+1) = {1/(r+1)} * 2(r²-1)(r+1)


Then I got

12r+12-r³-r² -r +1= 2r²-2

Then combined like terms

-r³-r²+11r+13=2r²-2

I subtracted (-r³-r²+11r+13) from both sides

r³+3r²-11r-15=0

Next I'm supposed to solve using the Quadratic Formula
x= (-b±√b²-4ac)/2a

Edit- Correct way

"(6/r²-1)-1/2= (1/r+1)"

{6/(r²-1)} * 2(r-1)(r+1) - (1/2) * 2(r-1)(r+1) = {1/(r+1)} * 2(r-1)(r+1)

12-r²+1=2r-2

r²+2r-15=0

r= {-2±√2²-4(1)(-15)}/2(1)

r= (-2±8)/2

r= 3,-5

Have you tried Math Homework Help - Answers to Math Problems - Hotmath

Check and see if your textbook is in there, and if it is, find the problem you're struggling on, and it'll probably go through it completely, for you.

I'm sorry if that didn't help, but that's what I always used--even in college.

v.v I could not find my textbook...

Hi there! I'm a university math major working on a certificate to teach high school math. So let's see if I can help.

The problem with your result is that it is a cubic, and you cannot use the quadratic formula to solve a cubic equation. Unfortunately when I went through this problem, I too got a cubic as a result. There is a formula for cubics, but I don't know if offhand. It is pretty ghastly though, and I don't think your instructor would expect you to use it.

I'll go through it again and see if there wasn't something I forgot to simplify, but first let's make sure I am setting up the problem correctly. I did get an entirely different cubic from yours, so maybe I've written it wrong.



Have I?

Originally Posted by Vox

Hi there! I'm a university math major working on a certificate to teach high school math. So let's see if I can help.

The problem with your result is that it is a cubic, and you cannot use the quadratic formula to solve a cubic equation. Unfortunately when I went through this problem, I too got a cubic as a result. There is a formula for cubics, but I don't know if offhand. It is pretty ghastly though, and I don't think your instructor would expect you to use it.

I'll go through it again and see if there wasn't something I forgot to simplify, but first let's make sure I am setting up the problem correctly. I did get an entirely different cubic from yours, so maybe I've written it wrong.



Have I?

Yes that's correct

I understand your calculations now. You have done the math correctly. The solution is a cubic. I am not sure what to tell you from here. This cannot be solved using the quadratic equation.

Originally Posted by Vox

I understand your calculations now. You have done the math correctly. The solution is a cubic. I am not sure what to tell you from here. This cannot be solved using the quadratic equation.

Could you tell me how to solve it otherwise? I can't really go to her for help because its a distance learning course and we watch the lectures online. Also I have no friends who are this high in math.

Very well. You can use the cubic equation, but I can promise you it will not be fun.

Originally Posted by Vox

Very well. You can use the cubic equation, but I can promise you it will not be fun.

:shock: That very does not look fun. Though if I can't come up with anything else it looks like I'm going to have to do that v.v

Yeah. Unless the problem has a typo or something. That's what I'd bank on, because I seriously doubt that a class at your level would even expect you to look at that eyesore of an equation much less use it.

Do you still need help with this problem? I think I want to take a crack at it o3o

Edit:

This should help get rid of that nasty cube!



I love algebra! Thanks for giving me something to do :lol:

Yep, that will do it. Nice job, Keyori. I swear I kept looking at that (r^2 -1) thinking, "There must be something I can do here... but (r + 1)^2 isn't it so... nahhh." I proved the Fundamental Theorem of Calculus today but I can't factor a polynomial. I need to get more fucking sleep.

I worked that problem three times before it dawned on me that I could factor it out easily, and I too was getting a cubic equation each time. So I'd definitely call it a relatively common mistake, and the teacher probably gave that problem specifically for what we did wrong. xD

I wish i would have seen this sooner... I <3 math and noticed the answer right away... >.>'

Thanks guys so much for the help!