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November 23, 2014

November 23, 2014

Posted by **Jake** on Saturday, April 14, 2012 at 7:56pm.

a) f(x) 4x^3+6x^2-24x-2

1. First to find intercepts

y=0

0=4x^3+6x^2-24x-2

=2(2x^3+3x^3-12x-1)

I don't know how to find the x-intercept. I can't use quadratic formula or synthetic division :S

- Calculus -
**Reiny**, Saturday, April 14, 2012 at 8:27pmok, so you would have to solve

4x^3 + 6x^2 - 24x - 2 = 0

I also could not find any "nice" f(x) = 0

(I tried x = ±2, ±1/2)

so I went to the reliable Wolfram to get

http://www.wolframalpha.com/input/?i=4x%5E3+%2B+6x%5E2+-+24x+-+2+%3D+0

notice that all 3 roots appear to be irrational

making it a messy solution.

- Calculus -
**Jake**, Saturday, April 14, 2012 at 9:15pmI checked in the book answer and the site u gave me matched with the answer.

How did the website get solutions as:

x= -3.28183

x= -1.863

- Calculus -
**Reiny**, Saturday, April 14, 2012 at 9:36pmThere is no simple formula to solve a general cubic equation.

There are several methods that can be used to find solutions ...

- Newton's Method and some type of iteration algorithms are the most popular

once you have one of the roots of the cubic, you can do long division or use synthetic division to reduce the cubic to a quadratic.

From there you can use the quadratic formula to find the other two roots

I don't know what method the Wolfram site uses.

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