Online Course Discussion Forum

Math Challenge II-A Algebra Final Quiz Problem 18

 
 
Picture of Tina Jin
Math Challenge II-A Algebra Final Quiz Problem 18
by Tina Jin - Sunday, 3 April 2022, 3:39 PM
 

Hello!

Can you help with problem 18, I got it wrong and the solution underneath the problem said it is the same as the previous problem but with y=1. The previous problem does not include y nor does it relate to this one.  How do you do the problem? Can you clarify a bit? Thank you so much!


Tina

 
Picture of John Lensmire
Re: Math Challenge II-A Algebra Final Quiz Problem 18
by John Lensmire - Tuesday, 5 April 2022, 11:17 AM
 

Hi Tina,

The final quiz actually has some problems from the previous chapters in it. This question, is actually 7.20, which references 7.19. I would recommend you take a look at the Chapter 7 practice quiz again and look at 7.19 and 7.20 there for the trick of factoring the solution mentions.

That being said, this question does amount to (after combining like terms and getting rid of the negatives) solving $x^3 + 3x^2 + x + 3 = 0$. Our hope when solving a general cubic is that we can find one rational root to help the factoring. The Rational Root Theorem can help. For this cubic, remember this theorem says that the only possible rational roots are $\pm 1$ and $\pm 3$. Trial and error here gives us that $x = -3$ is a root. Then factoring or polynomial long division gives us$$x^3 + 3x^2 + x + 3 = (x+3)(x^2+1)$$so $x=-3$ is the only real root.

Picture of Tina Jin
Re: Math Challenge II-A Algebra Final Quiz Problem 18
by Tina Jin - Thursday, 7 April 2022, 9:08 PM
 

Thank you!