cos2x的数值等于cos²x-sin²x=2cos²x-1=1-2sin²x=(1-tan²x)/(1+tan²x)

cos2x=cos²x-sin²x

cos2x=1-2sin²x

cos2x=2cos²x-1

cos2x=1-(1-2sin²x)=2sin²x

cos3a

=cos(2a+a)

=cos2acosa-sin^2asina

=(2cos^2a-1)cosa-2(1-cos^2a)cosa

=4cos^3a-3cosa

sin²α=[1-cos(2α)]/2

cos²α=[1+cos(2α)]/2

tan²α=[1-cos(2α)]/[1+cos(2α)]