Which complex number has a distance of the square root of 17 from the origin on the complex plane?

The complex plane is an interesting thing we use to graph complex numbers. The horizontal axis is where you plot the real part of the complex number, and the y-axis is where you plot the coefficient of the imaginary number.

For example, if I had the complex number 4+3i, I would plot the point (4,3) on the complex plane.

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MODULI

The modulus (plural is moduli) is the distance of a complex number from the origin on the complex plane. To find the modulus, you take the real number and the coefficient (basically the points of your graph), square them, add the results, and then you find the square root of that! Here's a little checklist so that you can always find the modulus of any complex number!

STEPS TO FIND MODULUS GIVEN COMPLEX NUMBER

STEP 1: PLOT THE COMPLEX NUMBER

Let's use our complex number 4+3i. If we plot it, the point will be (4,3)

STEP 2: TAKE YOUR VALUES OF X AND Y AND SQUARE THEM

4²=16

3²=9

STEP 3: ADD THESE SQUARES

16+9=25

STEP 4: SQUARE ROOT IT

√25=5

Therefore, the modulus (distance from the origin on the complex plane) of 4+3i is 5.

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SOLVING THE PROBLEM

In our case, we are given a modulus but not a point on the line. Our modulus is the square root of seventeen. We are going to have to the steps to find the modulus backwards to find our points. I will put the steps for finding a complex number given the modulus below.

STEPS TO FIND COMPLEX NUMBER GIVEN MODULUS

STEP 1: SQUARE THE MODULUS

√17²=17

STEP 2: FIND THE SQUARES THAT MAKE UP THE NUMBER

Before, we could just add things we found. Now, we have to find the things that were added! How do we know what things were added to find 17? Well, we know that we added two squares to find the modulus, so we will subtract a square of some sort and hopefully come up with another square.

Let's see what squares exist that are less than seventeen.

1, 4, 9, 16

(Note that one is 1², 4 is 2², 9 is 3², and 16 is 4²)

Which of these can we add to get 17?

Well, we can do 1 and 16, so this is our result so far.

STEP 3: SQUARE ROOT THE RESULTS

Now, we just find the square roots of what we found in the last problem!

√1=1

√16=4

STEP 4: PLOT THE GIVEN NUMBERS ON THE PLANE AND FIND THE COMPLEX NUMBER

We can plot these either way because the modulus of 1+4i will be the same distance from the origin as 4+i because we only use the commutative property (adding order doesn't matter) when looking for the modulus. You can also have the negative version of either number because you immediately square it, so it can also be -4+i or 4-i! You can also rearrange the equation in tons of ways to figure it out. You could also have both be negative, flip the coefficient placement, etc.!

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SOLUTION

Therefore, the complex numbers that have a distance of √17 from the origin on the complex plane are ±4±i and ±1±4i. So, why did I write all of these funny plus and minus signs? Well, it's so that I didn't have to write all of the possible equations. I believe those equations actually represent 8 possible points we can create with the solution we got.

4+i

-4+i

4-i

-4-i

1+4i

-1+4i

1-4i

-1-4i

You can plug these complex numbers back in and find the modulus with my instructions to find the modulus given a complex number. Now, you will be able to deal with moduli and complex numbers for the rest of your life!

I hope that this helps! Have a wonderful day! :)