cos2x等於多少

cos2x=cos2x-sin2x=2cos2x-1=1-2sin2x=(1-tan2x)/(1+tan2x)，即cos2x=1-2sinx的平方。

相關公式

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)]