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Algebra Level 4

A = { z : arg ( z ) = π 3 } B = { z : arg ( z 2 2 3 i ) = 2 π 3 } \begin{aligned} A&=\left \{z: \arg (z)=\frac{\pi}3 \right \}\\ B&=\left \{z: \arg \left(z-2-2\sqrt 3i \right)=\frac{2\pi}3 \right \} \end{aligned}

A A and B B are two sets defined as shown above.

Find the number of elements in A B A\cap B .

Notation: The function arg ( z ) \arg (z) denotes the argument of a complex number.

None of these 3 1 No intersection exists. 2

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1 solution

Let arg z \text{arg z} be the principal branch of argument, i.e, if a = 2 + 2 3 i = 4 e i π 3 = 4 e i π 3 + 2 π i = . . . a = 2 + 2\sqrt{3}i = 4\cdot e^{\frac{i\pi}{3}} = 4\cdot e^{\frac{i\pi }{3} + 2\pi i} = ... then arg a = π 3 \text{arg a} = \frac{\pi}{3} .

Then A A is the semi-line in the first quadrant in the Argand plane (cartesian plane) forming 60 º 60 º with real axis, i.e, A = { r e i π 3 ; r R , r > 0 } A = \{r\cdot e^{\frac{i\pi }{3}}; r \in \mathbb{R}, r >0\} and A B = ϕ A \cap B = \phi (the empty set) due to the sume of two vectors(apply the law of parallelogram), z C z \in \mathbb{C} and 2 2 3 i -2 - 2\sqrt{3}i will have an principal (main) branch of argument being 2 π 3 \frac{2\pi}{3} ,i.e, being B = { z = 2 + 2 3 i + r e 2 i π 3 ; r R , r > 0 } B = \{z = 2 + 2\sqrt{3}i + r\cdot e^{\frac{2i\pi }{3}}; r \in \mathbb{R}, r >0\} the semiline parallel to the semiline starting at the origin and forming 120 ° 120 ° with the real axis without cutting A A

Excellent solution.

Sravanth C. - 4 years, 6 months ago

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thank you very much, I have just tried to clarify my solution a bit

Guillermo Templado - 4 years, 6 months ago

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