Math

-x^3 + 1.85x^2 - x +0.15 = 0

how can you solve for x?

An exact solution or a numerical solution?

Exact solutions can be obtained as follows:

x^3 - 1.85x^2 + x - 0.15 = 0

First get rid of the quadratic term by substituting:

x = (y + 1.85/3)

You then obtain an equation of the form:

y^3 + p y + q = 0

Write this as:

y^3 = -p y - q

Compare this to the formula for (a+b)^3 which can be written as:

(a+b)^3 = 3 a b (a+b) + a^3 + b^3

This looks very similar to the equation you want to solve with a + b playing the role of y. If you can somehow choose a and b such that:

3 a b = -p (1)

and

a^3 + b^3 = -q (2)


then the equation becomes exactly similar and the solution would thus be

y = a + b

But how do you choose a and b such that Eqations (1) and (2) are satisfied? If you take the third power of equation (1), you get:

27 a^3 b^3 = -p^3

if we put A = a^3 and B = b^3, then we have:

A B = -p^3/27

and (2) implies that:

A + B = -q

Solving for A and B is easy, if you use one equation to eliminate, say A and substitute that in the other, you get a quadratic equation. So, all you have to do is to solve that quadratic equation, calculate a and b from that.

if i already know x-1 is a factor, how can i solve it from here?

First of all multiply each term by -1 to start your equation with a positive term.

notice that the coefficients of the positive terms add up to 2 and the coefficients of the negative terms add up to -2, so x=1 is a solution (this was luck)
Divide the cubic by x-1 to get x^2 - .85x + .15 = 0
which factors to (x-.6)(x-.25)=0

So you have 3 rational solutions:
x=1, x=.6, and x=.25

If you know one of the factors, then divide the function by that factor.

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asked by JV

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