Question

Use demoivre's theorem to show that;
cos4¤ =8 cos4¤ -8cos2¤+1​

Answer 1

`\cos4( \theta)=Re(\cos4( \theta)+i\sin4( \theta)) \\=Re(\cos ( \theta)+i\sin ( \theta))^4 \\ =Re(cos^4( \theta)+4cos^3( \theta)isin( \theta)+6cos^2( \theta)i^2sin^2( \theta)+4cosi^3( \theta)sin^3( \theta)+i^4sin^4( \theta)) \\cos( 4\theta) =cos^4( \theta)-6cos^2( \theta)sin^2( \theta)+sin^4( \theta) \\ =(1-\sin^2( \theta))^2-6(1-\sin^2( \theta))\sin^2( \theta)+sin^4( \theta) \\ = 1-2\sin^2( \theta)+\sin^4( \theta)-6\sin^2( \theta)+6\sin^4( \theta)+\sin^4( \theta) \\ =8\sin^4( \theta)-8\sin^2( \theta)+1`