Math Problem Statement

Consider the following complex number: w = (1 - √3i)^5 / (1 + i)^4. (a) Express w in Cartesian form and in polar form.

Solution

Let's analyze the complex number ww given by the expression:

w=(13i)5(1+i)4w = \frac{(1 - \sqrt{3}i)^5}{(1 + i)^4}

Step 1: Express in Polar Form

Denominator: 1+i1 + i

To express 1+i1 + i in polar form, we calculate its modulus rr and argument θ\theta.

  • Modulus: r=1+i=12+12=2r = |1 + i| = \sqrt{1^2 + 1^2} = \sqrt{2}

  • Argument: θ=tan1(11)=π4\theta = \tan^{-1}\left(\frac{1}{1}\right) = \frac{\pi}{4}

Thus, in polar form: 1+i=2cisπ41 + i = \sqrt{2} \text{cis} \frac{\pi}{4} where "cis" stands for cosθ+isinθ\cos \theta + i \sin \theta.

Now raise this to the 4th power: (1+i)4=(2)4cis(4×π4)=4cisπ=4(1 + i)^4 = (\sqrt{2})^4 \text{cis} \left(4 \times \frac{\pi}{4}\right) = 4 \text{cis} \pi = -4

Numerator: 13i1 - \sqrt{3}i

Similarly, express 13i1 - \sqrt{3}i in polar form.

  • Modulus: r=13i=12+(3)2=4=2r = |1 - \sqrt{3}i| = \sqrt{1^2 + (\sqrt{3})^2} = \sqrt{4} = 2

  • Argument: θ=tan1(31)=π3\theta = \tan^{-1}\left(\frac{-\sqrt{3}}{1}\right) = -\frac{\pi}{3}

Thus, in polar form: 13i=2cis(π3)1 - \sqrt{3}i = 2 \text{cis} \left(-\frac{\pi}{3}\right)

Now raise this to the 5th power: (13i)5=25cis(5×(π3))=32cis(5π3)(1 - \sqrt{3}i)^5 = 2^5 \text{cis} \left(5 \times \left(-\frac{\pi}{3}\right)\right) = 32 \text{cis} \left(-\frac{5\pi}{3}\right)

Step 2: Combine to Find ww

Now, combine the numerator and denominator:

= -8 \text{cis} \left(-\frac{5\pi}{3} - \pi\right) = -8 \text{cis} \left(-\frac{8\pi}{3}\right)$$ Since the argument $$-\frac{8\pi}{3}$$ is coterminal with $$\frac{4\pi}{3}$$, we have: $$w = -8 \text{cis} \frac{4\pi}{3}$$ Thus, the polar form of $$w$$ is: $$w = 8 \text{cis} \left( \frac{4\pi}{3} \right)$$ ### Step 3: Convert to Cartesian Form To convert back to Cartesian form, use the definition of "cis": $$w = 8 \left( \cos \frac{4\pi}{3} + i \sin \frac{4\pi}{3} \right)$$ We know: $$\cos \frac{4\pi}{3} = -\frac{1}{2}, \quad \sin \frac{4\pi}{3} = -\frac{\sqrt{3}}{2}$$ Thus: $$w = 8 \left( -\frac{1}{2} - i \frac{\sqrt{3}}{2} \right) = -4 - 4\sqrt{3}i$$ ### Final Answer: - **Polar form:** $$w = 8 \text{cis} \left( \frac{4\pi}{3} \right)$$ - **Cartesian form:** $$w = -4 - 4\sqrt{3}i$$ Would you like further details or have any specific questions? Here are 5 questions related to this problem: 1. How do you calculate the modulus and argument of a complex number? 2. Why do we use polar form to simplify powers of complex numbers? 3. What does "cis" mean in polar form? 4. How do arguments change when raising a complex number to a power? 5. Why is the argument in the final polar form expressed as $$\frac{4\pi}{3}$$? **Tip:** When converting between Cartesian and polar forms, always verify your calculations of the modulus and argument to ensure accuracy.

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Math Problem Analysis

Mathematical Concepts

Complex Numbers
Polar Coordinates
Powers of Complex Numbers

Formulas

Polar form: z = r(cos θ + i sin θ) or r cis θ
Modulus: |z| = √(a^2 + b^2) for z = a + bi
Argument: θ = tan⁻¹(b/a) for z = a + bi
Multiplication and exponentiation of complex numbers in polar form

Theorems

De Moivre's Theorem

Suitable Grade Level

Undergraduate Mathematics