Finding a Complex Number Given Arg and Modulus

In summary, the argument of w must be equal to pi/4 and the product of w and its conjugate must be equal to 20. This can only be achieved when a and b are equal and are positive, resulting in w being equal to the square root of 10 plus the square root of 10i. Negative values for a and b would result in w being in the wrong quadrant.
  • #1
squenshl
479
4

Homework Statement


If ##\text{arg}(w)=\frac{\pi}{4}## and ##|w\cdot \bar{w}|=20##, then what is ##w## of the form ##a+bi##.

Homework Equations

The Attempt at a Solution


The only way for the argument of ##w## to be ##\frac{\pi}{4}## is when ##a+bi## where ##a=b \in \mathbb{Z}## right?
 
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  • #3
andrewkirk said:
That is correct.
Great thanks.
I get ##a=b= \pm\sqrt{10}## so ##w## follows.
 
  • #4
squenshl said:
Great thanks.
I get ##a=b= \pm\sqrt{10}## so ##w## follows.

No, ##a = b = -\sqrt{10}## would not be correct; only the ##+\sqrt{10}## answer applies.
 
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  • #5
Ray Vickson said:
No, ##a = b = -\sqrt{10}## would not be correct; only the ##+\sqrt{10}## answer applies.

Do you understand Ray's point? What is ##\arg(w)## if ##a = b## and both are negative?
 
  • #6
RPinPA said:
Do you understand Ray's point? What is ##\arg(w)## if ##a = b## and both are negative?
I do.
##w=-\sqrt{10}-\sqrt{10}i## is in the wrong quadrant.
##\text{arg}(w)=-\pi+\frac{\pi}{4}##.
 

1. What is a complex number?

A complex number is a number that contains both a real and an imaginary part. It is expressed in the form a + bi, where a is the real part, b is the imaginary part, and i is the imaginary unit equal to the square root of -1.

2. What is the modulus of a complex number?

The modulus of a complex number is the distance of the number from the origin on the complex plane. It is calculated by taking the square root of the sum of the squares of the real and imaginary parts.

3. What is the argument of a complex number?

The argument of a complex number is the angle between the positive real axis and the line connecting the number to the origin on the complex plane. It is measured in radians or degrees and can be calculated using trigonometric functions.

4. How do you find a complex number given its argument and modulus?

To find a complex number given its argument and modulus, you can use the polar form of complex numbers. The real part of the number is equal to the modulus multiplied by the cosine of the argument, and the imaginary part is equal to the modulus multiplied by the sine of the argument.

5. Can a complex number have multiple values for its argument and modulus?

Yes, a complex number can have multiple values for its argument and modulus, as there are an infinite number of ways to represent the same number on the complex plane. These different representations are known as equivalent forms of the complex number.

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