Factoring a polynomial where factor theorem doesn't work

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SUMMARY

The polynomial equation x^3 - 9x^2 + 15x + 30 cannot be factored using the factor theorem due to the absence of rational roots. The discussion highlights the use of a graphing calculator to approximate a root near -1.13 and suggests applying Newton's method for numerical approximation. For an exact solution, the cubic formula is recommended as an alternative approach.

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  • Knowledge of the cubic formula for solving cubic equations
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Homework Statement



Solve: x^3 - 9x^2 + 15x + 30

Homework Equations





The Attempt at a Solution



The factors of 30 are +-1, +-2, +-3, +-5, +-6, +-10, +-15, and +-30.

I used my graphing calculator and got a zero close to -1. I plugged it into the original equation and got 5, not 0. I used the zero function on my calculator and found that the zero occurred around -1.13. that's a fraction.

How would I solve this algebraically?
 
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Try Newtons method:
Define
[tex]T(x)=x-\frac{f(x)}{f'x}=x-\frac{x^3-9x^2+15x+30}{3x^2+18x+15}[/tex]
 
If you are only looking for a numerical approximation, then Newton's method should work fine for you. However, if you are looking for an exact analytical solution, try the cubic formula
 

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