1、sin2α=2sinαcosα.

2、tan2α=2tanα/( 1-tan^2(α))。

3、cos2α=cos^2(α)-sin^2(α)=2cos^2(α)- 1= 1-2sin^2(α).

4、sin^2(α/2)=( 1-cosα)/2。

5、cos^2(α/2)=( 1+cosα)/2。

6、tan^2(α/2)=( 1-cosα)/( 1+cosα)。

7、tan(α/2)= sinα/( 1+cosα)=( 1-cosα)/sinα.

8. The double angle formula represents the trigonometric function value of the double angle 2α through some transformation relations of the trigonometric function value of the angle α. The angle doubling formula includes sine angle doubling formula, cosine angle doubling formula and tangent angle doubling formula.