Question 870251
use an angle sum identity to verify the identity cos 2 theta = 2 cos^2 theta -1
{{{cos(2theta)=cos(theta+theta)=cos(theta)*cos(theta)-sin(theta)*sin(theta)=cos^2(theta)-sin^2(theta)=cos^2(theta)-(1-cos^2(theta))=2cos^2(theta)-1}}}