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Which polynomial function has a leading coefficient of 1 and roots [tex]\(2i\)[/tex] and [tex]\(3i\)[/tex] with multiplicity 1?

A. [tex]\(f(x) = (x - 2i)(x - 3i)\)[/tex]

B. [tex]\(f(x) = (x + 2i)(x + 3i)\)[/tex]

C. [tex]\(f(x) = (x - 2)(x - 3)(x - 2i)(x - 3i)\)[/tex]

D. [tex]\(f(x) = (x + 2i)(x + 3i)(x - 2i)(x - 3i)\)[/tex]


Sagot :

To find the polynomial function with a leading coefficient of 1 and roots [tex]\(2i\)[/tex] and [tex]\(3i\)[/tex] each with a multiplicity of 1, we follow these steps:

1. Identify the given roots: The roots of the polynomial are [tex]\(2i\)[/tex] and [tex]\(3i\)[/tex] each with a multiplicity of 1. Roots also come with their complex conjugates, thus the full set of roots is [tex]\(\{2i, -2i, 3i, -3i\}\)[/tex].

2. Form the factors of the polynomial: For each root [tex]\(r\)[/tex], the corresponding factor is [tex]\((x - r)\)[/tex]. Therefore, the factors are:
[tex]\[ (x - 2i), (x + 2i), (x - 3i), (x + 3i) \][/tex]

3. Construct the polynomial by multiplying these factors:
[tex]\[ f(x) = (x - 2i)(x + 2i)(x - 3i)(x + 3i) \][/tex]

4. Simplify pairs of complex conjugate factors:
[tex]\[ (x - 2i)(x + 2i) = x^2 - (2i)^2 = x^2 - (-4) = x^2 + 4 \][/tex]
Similarly,
[tex]\[ (x - 3i)(x + 3i) = x^2 - (3i)^2 = x^2 - (-9) = x^2 + 9 \][/tex]

5. Multiply the resulting quadratic polynomials:
[tex]\[ f(x) = (x^2 + 4)(x^2 + 9) \][/tex]

6. Expand the polynomial using the distributive property:
[tex]\[ f(x) = x^2(x^2 + 9) + 4(x^2 + 9) \][/tex]
[tex]\[ = x^4 + 9x^2 + 4x^2 + 36 \][/tex]
[tex]\[ = x^4 + 13x^2 + 36 \][/tex]

Therefore, the polynomial function with a leading coefficient of 1 and roots [tex]\(2i\)[/tex] and [tex]\(3i\)[/tex] each with a multiplicity of 1 is:
[tex]\[ f(x) = x^4 + 13x^2 + 36 \][/tex]

This matches with the option:
[tex]\[ \boxed{(x + 2i)(x + 3i)(x - 2i)(x - 3i)} \][/tex]
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