- #1
aeromat
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Homework Statement
Find the zeros of the cubic equation:
[tex]y = x^3 -9x^2 + 15x + 30[/tex]
How do we find the zeros of this? In this case, subbing in x-values that will make it equal 0 does not work.
gb7nash said:I would probably suggest using Newton's method (or bisection method if you can find a positive function value and a negative function value) Once you've found the first root, take f(x)/(x-r), where r is the root. This will yield a quadratic and you can use the quadratic formula to find the remaining two roots.
gb7nash said:I'm missing something here. How do we know there's only one real root?
Dick said:You can also conclude there is only one root by looking at the derivative and finding the extreme values plus knowing the behavior as x->+/-infinity. Which is basically 'graphing it' without a calculator. Hence, not cheating.
The simplest way to find zeros of a cubic equation when the rational roots test fails is by using the graphing method. This involves plotting the equation on a graph and visually determining the x-intercepts, which represent the zeros of the equation.
Yes, the graphing method can be used for all cubic equations. However, it may not always be the most efficient or accurate method. Other methods such as the factoring method or the Newton-Raphson method may be more suitable depending on the specific equation.
Yes, the graphing method can still be used for cubic equations with complex roots. However, the graph may be more difficult to interpret and additional calculations may be needed to determine the exact values of the complex roots.
One limitation of the graphing method is that it may not be accurate for equations with multiple or repeated roots. In these cases, other methods such as the quadratic formula or the Cubic Formula may be more effective.
Yes, technology such as graphing calculators or online graphing tools can be used to plot the equation and determine the x-intercepts. This can make the process more efficient and accurate.