Using de Moivre's theorem demonstrate that "sin6x+sin2x(16(sinx)^4-16(sinx)^2+3)"

Consider, (cosX+isinX)6 using binomial expansion we find that this = "cos6X + 6icos5XsinX - 15cos4Xsin2X - 20icos3Xsin3X + 15cos2Xsin4X + 6icosXsin5X - sin6X". Next, by using de Moivre's theorem (r(cos(X)+isin(X)))n = rn(cos(nX)+isin(nX)) we find that (cosX+isinX)6 is also = cos6X+isin6X. So in a brief summary the whole binomial expansion is = to cos6X+isin6X.Now, by looking at the imaginary parts of both sides we can see that "isin6X = 6icos5XsinX - 20icos3Xsin3X + 6icosXsin5X" I'm seeing some lovely factors that I'd love to pull out, so lets! (I've also removed the i as it was common on both sides)"sin6X = 2cosXsinX(3cos4X - 10cos2Xsin2X + 3sin4X)" by using our trig identities we know that 2cosXsinX = sin2X and that cos2X = 1 - sin2X so, "sin6X = sin2X(3(1 - sin2X)2 - 10(1 - sin2X)sin2X + 3sin4X)" Expanding the brackets we end up with "sin6X = sin2X( 3 - 6sin2X + 3sin4X - 10sin2X + 10sin4X + 3sin4X)" which leaves us to our final line! that "sin6X = sin2X( 3 + 16sin4X - 16sin2X)" proved using de Moire's theorem

Answered by Further Mathematics tutor

3953 Views

See similar Further Mathematics A Level tutors

Related Further Mathematics A Level answers

All answers ▸

The curve C has parametric equations x=cos(t)+1/2*sin(2t) and y =-(1+sin(t)) for 0<=t<=2π. Find a Cartesian equation for C. Find the volume of the solid of revolution of C about the y-axis.


Find the square root of i


Find the eigenvalues and eigenvectors of A = ([2, 0 , 0], [0, 1, 1], [0, 3, 3])


Given M = [[-2,6],[1,3]], find P and D such that M = PDP^(-1) where D is a diagonal matrix


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

© MyTutorWeb Ltd 2013–2025

Terms & Conditions|Privacy Policy
Cookie Preferences