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Polynomials .Find the polynomial with lowest degree having the following zeros : 2 and...

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fairydrink | Student, Grade 10 | (Level 2) Honors

Posted May 10, 2011 at 3:10 AM via web

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Polynomials .

Find the polynomial with lowest degree having the following zeros : 2 and 2i .

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

Posted May 10, 2011 at 10:23 AM (Answer #2)

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By definition, all zeroes of a polynomial are, in fact, the roots of that polynomial. According to the rule, the complex solutions come in complex conjugate pairs, therefore if 2i is a solution of the equation, it's conjugate -2i is also the solution of the equation.

We'll start from the fact that a polynomial could be written as a product of linear factors, if we know it's zeroes.

x1 = 2

x2 = 2i

x3 = -2i

The polynomial is:

P(x) = a(x - x1)(x - x2)(x - x3)

We'll substitute x1,x2 and x3:

P(x) = (x - 2)(x - 2i)(x + 2i)

We'll write the product (x - 2i)(x + 2i) as a difference of squares:

(x - 2i)(x + 2i) = x^2 + 4

P(x) = (x - 2)(x^2 + 4)

We'll remove the brackets:

P(x) = x^3 + 4x - 2x^2 - 8

The lowest degree polynomial, having as roots the values 2, 2i, -2i, is: P(x) = x^3 - 2x^2 + 4x - 8.

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted May 11, 2011 at 5:36 AM (Answer #3)

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A polynomial has complex roots in pairs of conjugates. As the polynomial has roots 2 and 2i, it also has -2i as a root.

The polynomial is: (x - 2)(x - 2i)(x + 2i)

=> (x - 2)(x^2 - 4i^2)

=> (x - 2)(x^2 + 4)

=> x^3 - 2x^2 + 4x - 8

The required polynomial is x^3 - 2x^2 + 4x - 8

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