DeMoivre's FormulaDate: 08/13/98 at 05:18:23 From: Greg Lukens Subject: Demove's Formula I have forgotten about DeMoivre's formula. Thanks in advance for the reminder. Greg Lukens Date: 08/13/98 at 14:07:29 From: Doctor Benway Subject: Re: Demove's formula Hi Greg, I believe the formula you're looking for is DeMoivre's formula, which is the following: (cos(theta) + i*sin(theta))^n = cos(n*theta) + i*sin(n*theta) This formula is useful when you have a complex number and want to raise it to some power without doing a lot of work. If all you want is the formula, you can ignore the rest of this message. However, if you want a little more insight into what is going on, read on. Recall that any complex number can be written in the form r*e^(i*theta). If you plot a complex number in the complex plane (where the x-axis is the real axis and y-axis is the imaginary axis), then "r" will be the distance from the point to the origin and theta will be the angle a line from the origin to the point makes with the x-axis. A little trig shows that a complex number written as r*e^(i*theta) can also be written as r*cos(theta)+r*i*sin(theta). Knowing this little fact gives us the ability to switch back and forth between ways of writing complex numbers, depending on what we want to do with them. If we want to add complex numbers, then the form a + b*i is easiest, whereas if we want to multiply them together, it is easier to use the form r*e^(i*theta). Essentially what you are doing is taking a complex number of the form (a + b*i), converting it to the form r*e^(i*theta), raising it to a power in that form, then converting back to the first form. Observe: (r*cos(theta) + r*i*sin(theta))^n = (r*(cos(theta) + i*sin(theta))^n = (r^n) * (cos(theta) + i*sin(theta))^n = (r^n) * (e^(i*theta))^n = (r^n) * (e^(n*i*theta)) = (r^n) * (cos(n*theta) + i*sin(n*theta)) Of course knowing DeMoivre's formula allows us to go straight from (r*(cos(theta) + i*sin(theta))^n to (r^n) * (cos(n*theta) + i*sin(n*theta)). Thanks for writing, hope this helps. - Doctor Benway, The Math Forum Check out our web site! http://mathforum.org/dr.math/ Date: 08/13/98 at 16:55:10 From: Greg Lukens Subject: Re: Demove's formula Hi Dr. Math, That was way more than I was looking for. I found it quite interesting and am glad to know it. Thanks for your help, Greg |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994-2015 The Math Forum
http://mathforum.org/dr.math/