Math x-int's

How do I algebraically find the x-intercepts for this equation y = x^3 - 3x^2 + 3.

I know I need to plug in y = 0 to solve for x.

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

But where do I go from there? I don't think I can factor, and I don't think I can use the quadratic formula.

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asked by jeff
  1. It is actually very simple. To solve an equation of the form:

    x^3 + a x^2 + b x + c = 0

    you follow the following steps.

    First, get rid of the quadratic term
    a x^2 using the substitution:

    x = y - a/3


    This step is analogous to how you solve the quadratic equation when you write it as a perfect square.

    In this case we aren't finshed yet as the equation now is of the form:

    y^3 + p y + q = 0


    How do we solve this equation?

    Consider the identity:

    (a+b)^3 = a^3 + 3a^2b + 3ab^2 + b^3

    We can rewrite the right hand side as follows:

    a^3 + 3a^2b + 3ab^2 + b^3 =

    a^3 + b^3 + 3ab(a+b)

    So, we have:

    (a+b)^3 = 3ab(a+b)+ a^3 + b^3 ---->

    (a+b)^3 - 3ab(a+b) - [a^3+b^3] = 0

    Of course, this equation is always satisfied whatever values for a and b you substitute in it. Now, if we choose a and b such that:

    3ab = -p (1)

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

    Then:

    (a+b)^3 +p(a+b) + q = 0

    so y = a + b is then the solution we are looking for.

    So, it all boils down to solving equations (1) and (2) for a and b.

    If you thake the third power of (1):


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

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

    A*B = -p^3/27

    A + B = -q

    If you eliminate, say, B , you get a quadratic equation for A which you know how to solve. You then only need to pay attention when you extract the cube roots to find a and b. Equation (1) has to be satisfied, which means that if you choose one of the three possible complex cube roots for A, the root for B is fixed.

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  2. So, let's see how this works out for in your case:

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

    We put x = y + 1, which gives after some simple algebra:

    y^3 - 3y + 1 = 0

    So, p = -3 and q = 1

    We need to solve:

    A*B = -p^3/27 = 1

    A + B = -q = -1

    The second equation says that
    B = -(1+A)

    Substituting in the first gives:

    -A^2 - A = 1 --->

    A^2 + A + 1 = 0 ----->

    A = -1/2 ± i sqrt[3]/2

    We have two solutions because interchanging A and B will give you another solution. But if you do that the other solution for A will be the first solution for B. So we can take:

    A = -1/2 + i sqrt[3]/2

    B = -1/2 - i sqrt[3]/2

    We can rewrite this as:

    A = cos(4 pi/3) - i sin(4 pi/3)

    B = cos(4 pi/3) + i sin(4 pi/3)

    Which we can also write as:

    A = exp(-4 pi i/3)

    B = exp(4 pi i/3)

    Which means that:

    a = exp(-4 pi i/9 + 2 pi i n/3)

    b = exp(4 pi i/9 - 2 pi i n/3)

    Note that the product of a and b must be 1, the integer n which selects which of the three complex cube roots we choose for A fixes the cube root for B.

    The solution is thus:

    y = a + b =

    exp(-4 pi i/9 + 2 pi i n/3)

    + exp(4 pi i/9 - 2 pi i n/3) =

    2 cos(4 pi/9 + 2 pi n/3)

    And

    x = y + 1 = 2 cos(4 pi/9 + 2 pi n/3)

    The three solutions are thus:

    x = 2 cos(4 pi/9) + 1

    x = 2 cos(10 pi/9) + 1

    x = 2 cos(2 pi/9) + 1

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  3. I guess we will have to find one of the roots by brute force.
    We are looking for zeros of f(x) for some x
    Make a table to try to find the zeros
    for example
    x f(x)
    0 +3
    1 +1
    2 -1
    3 +3
    That is interesting, f(x) goes through zero at least once between x = +1 and x +2. Try x = 1.3
    1.3 +1.27
    1.4 -.136
    Oh, so between 1.3 and 1.4
    Try 1.33
    1.33 +.045937
    1.34 +.019304
    1.35 -.007125
    Humm, I am not going do go any further. That is close enough for now. The function crosses the x axis at x is about 1.35
    Now what. I need to factor that root out so I am left with a quadratic or else continue graphing and calculating to find the other two roots.
    To factor it out, note that (x-1.35) is a factor (to our level of approximation) so
    (x-1.35)(a x^2 + b x +c) = x^3-3x^2+3
    so we need to divide the right by (x-1.35)
    I get
    x^2 - 1.65 x - 2.2275
    solve that quadratic to find the other two roots.
    However I bet the problem is a typo unless you have covered this stuff in class.

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    posted by Damon
  4. Wow... that's a long process. Thanks for the help!

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    posted by jeff
  5. ivjlllllllllllllllllllllllllllllhjjjjjjlhlh

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