How can I solve a cubic equation in terms of a variable?

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In summary, if you want to find the roots of S in terms of K for the equation S3+101S2+(10K+100)S+100K, you can use Cardano's method for an exact solution. However, the cubic formula can be messy. Another option is to factor the equation and use the trigonometric form, which simplifies it to a quadratic. This form has solutions for t that can be converted to solutions for S in terms of K.
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ollie456
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Can anyone help solve S3+101S2+(10K+100)S+100K to find the roots of S in terms of K
 
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If you want an exact solution (i.e. not a numerical approximation) then you will have to use Cardano's method. The cubic formula is a bit messy in its generality though.
http://en.wikipedia.org/wiki/Cubic_function#Roots_of_a_cubic_function

You could also try guessing some obvious solutions and factor them, which will simplify the cubic to a quadratic. But I didn't see any just by glancing at the equation.
 
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Related to How can I solve a cubic equation in terms of a variable?

1. What is a cubic equation?

A cubic equation is a polynomial equation of the form ax^3 + bx^2 + cx + d = 0, where a, b, c, and d are constants and x is the variable. It is called a cubic equation because it involves a variable raised to the third power (cubed).

2. How do you solve a cubic equation?

There are several methods for solving a cubic equation, including the rational root theorem, synthetic division, and the cubic formula. The most commonly used method is the cubic formula, which involves finding the roots (or solutions) of the equation by using a specific formula.

3. What is the cubic formula?

The cubic formula, also known as the Cardano formula, is a method for finding the roots of a cubic equation. It involves finding the roots of a depressed cubic equation (one in which the coefficient of the x^2 term is zero) and then using those roots to find the roots of the original equation. The formula is quite complex and involves the use of complex numbers.

4. Are there any special cases when solving a cubic equation?

Yes, there are two special cases when solving a cubic equation: when the equation has a double root (two of the three roots are equal) and when the equation has three real roots. In the case of a double root, the cubic formula can be simplified and in the case of three real roots, the roots can be found using the rational root theorem.

5. Can all cubic equations be solved?

No, not all cubic equations can be solved using the cubic formula. Some cubic equations have irrational (non-repeating, non-terminating) solutions that cannot be expressed using radicals (square roots, cube roots, etc.). These equations are known as irreducible cubics and cannot be solved using traditional methods.

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