Quotient relation of trigonometric function: tan α = sin α/cos α; cotα=cosα/sinα.

Square relation: sin? α+cos？ α= 1;

1+ Tan? α = seconds? α; 1+cot？ α=csc？ α。

Half-angle formula of trigonometric function

sin(α/2)= √(( 1-cosα)/2)

cos(α/2)= √(( 1+cosα)/2)

tan(α/2)=√(( 1-cosα)/(( 1+cosα))？

General formula of trigonometric function

sin(α)=[2tαn(α/2)]/[ 1+tαN2(α/2)]

cos(α)=[ 1-tαN2(α/2)]/[ 1+tαN2(α/2)]

tαn(α)=[2tαn(α/2)]/[ 1-tαN2(α/2)]？

Double angle formula of trigonometric function

Sin2A=2SinA*CosA

cos2a=cosa^2-sina^2= 1-2sina^2=2cosa^2- 1

tan2A=(2tanA)/( 1-tanA^2)