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Which complex number has an absolute value of 5?

A. [tex]\(-3 + 4i\)[/tex]
B. [tex]\(2 + 3i\)[/tex]
C. [tex]\(7 - 2i\)[/tex]
D. [tex]\(9 + 4i\)[/tex]


Sagot :

To determine which of the following complex numbers has an absolute value (or modulus) of 5, we must first understand how to calculate the absolute value of a complex number. For a complex number [tex]\(a + bi\)[/tex], its absolute value is calculated as follows:

[tex]\[ |a + bi| = \sqrt{a^2 + b^2} \][/tex]

Let's evaluate each given complex number step-by-step.

1. For the complex number [tex]\(-3 + 4i\)[/tex]:
[tex]\[ | -3 + 4i | = \sqrt{(-3)^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5 \][/tex]
So, the absolute value of [tex]\(-3 + 4i\)[/tex] is 5.

2. For the complex number [tex]\(2 + 3i\)[/tex]:
[tex]\[ | 2 + 3i | = \sqrt{2^2 + 3^2} = \sqrt{4 + 9} = \sqrt{13} \approx 3.6 \][/tex]
The absolute value of [tex]\(2 + 3i\)[/tex] is approximately 3.6, which is not 5.

3. For the complex number [tex]\(7 - 2i\)[/tex]:
[tex]\[ | 7 - 2i | = \sqrt{7^2 + (-2)^2} = \sqrt{49 + 4} = \sqrt{53} \approx 7.28 \][/tex]
The absolute value of [tex]\(7 - 2i\)[/tex] is approximately 7.28, which is not 5.

4. For the complex number [tex]\(9 + 4i\)[/tex]:
[tex]\[ | 9 + 4i | = \sqrt{9^2 + 4^2} = \sqrt{81 + 16} = \sqrt{97} \approx 9.85 \][/tex]
The absolute value of [tex]\(9 + 4i\)[/tex] is approximately 9.85, which is not 5.

Given these calculations, the complex number [tex]\(-3 + 4i\)[/tex] has an absolute value of 5.