How to Write Polynomial Functions When Given Zeros

The zeros of a polynomial can be found by factoring.
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The zeros of a polynomial function of x are the values of x that make the function zero. For example, the polynomial x^3 - 4x^2 + 5x - 2 has zeros x = 1 and x = 2. When x = 1 or 2, the polynomial equals zero. One way to find the zeros of a polynomial is to write in its factored form. The polynomial x^3 - 4x^2 + 5x - 2 can be written as (x - 1)(x - 1)(x - 2) or ((x - 1)^2)(x - 2). Just by looking at the factors, you can tell that setting x = 1 or x = 2 will make the polynomial zero. Notice that the factor x - 1 occurs twice. Another way to say this is that the multiplicity of the factor is 2. Given the zeros of a polynomial, you can very easily write it -- first in its factored form and then in the standard form.

    Subtract the first zero from x and enclose it in parentheses. This is the first factor. For example if a polynomial has a zero that is -1, the corresponding factor is x - (-1) = x + 1.

    Raise the factor to the power of the multiplicity. For instance, if the zero -1 in the example has a multiplicity of two, write the factor as (x + 1)^2.

    Repeat Steps 1 and 2 with the other zeros and add them as further factors. For instance, if the example polynomial has two more zeros, -2 and 3, both with multiplicity 1, two more factors -- (x + 2) and (x - 3) -- must be added to the polynomial. The final form of the polynomial is then ((x + 1)^2)(x + 2)(x - 3).

    Multiply out all the factors using the FOIL (First Outer Inner Last) method to get the polynomial in the standard form. In the example, first multiply (x + 2)(x - 3) to get x^2 + 2x - 3x - 6 = x^2 - x - 6. Then multiply this with another factor (x + 1) to get (x^2 - x - 6)(x + 1) = x^3 +x^2 - x^2 - x - 6x - 6 = x^3 - 7x - 6. Finally, multiply this with the last factor (x + 1) to get (x^3 - 7x - 6)(x + 1) = x^4 + x^3 -7x^2 - 7x - 6x - 6 = x^4 + x^3 - 7x^2 - 13x - 6. This is the standard form of the polynomial.

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