# Checking a Root of a Cubic

• Sep 7th 2011, 05:52 AM
danthegreat
Checking a Root of a Cubic
So I have this math problem...

is -6 a root of....
x^3-5x^2-3x-18=0? yes or no?

i plug this equation in my calculator and go to the table. It says the coordinates are (-6,0) which means when I plug in -6, it should equal 0... However, when I do the work, I end up getting -183...

(-6)^3-5(-6)^2-3(-6)-18=0
-216 -5 +36 +18 - 18 =0

-216-5+36 = -183 @_@?

am I doing something wrong?
• Sep 7th 2011, 06:15 AM
re: Checking a Root of a Cubic

I get positive 6 as an answer:

6^3 - 5(6^2)-3(6)-18=0

216-180-36-18=0
• Sep 7th 2011, 06:15 AM
Plato
re: Checking a Root of a Cubic
Quote:

Originally Posted by danthegreat
So I have this math problem...

is -6 a root of....
x^3-5x^2-3x-18=0? yes or no?

i plug this equation in my calculator and go to the table. It says the coordinates are (-6,0) which means when I plug in -6, it should equal 0... However, when I do the work, I end up getting -183...

(-6)^3-5(-6)^2-3(-6)-18=0
-216 -5 +36 +18 - 18 =0

-216-5+36 = -183 @_@?

Where did that come from?
• Sep 7th 2011, 06:19 AM
danthegreat
re: Checking a Root of a Cubic
Oh wow...I am one huge dummy. (Giggle)

I checked the coordinates (6,0), not -6. I rechecked and got (-6,-396) ??? Still doesn't make sense..

Plato --

(-6)^3-5(-6)^2-3(-6)-18=0
-216 -5 +36 +18 - 18 =0

-5 is brought down from above and then 36 is the result of -6 squared.

EDIT: Figured out the problem stated above....Thanks! I forgot to multiply 5*36 and instead added them.