Doppelter winkel
`sin2α = 2sin α * cos α`
`sin2α = (2 tan α)/(1+tan^2α)=(2 cot α)/(1+cot^2α)=2/(tan α + cot α)`
`cos2α= cos^2α-sin^2α=2cos^2α-1 = 1-2sin^2α`
`cos2α = (1-tan^2α)/(1+tan^2α)=(cot^2α-1)/(cot^2α+1)=(cot α-tan α)/(cot α + tan α)`
`tan 2α = (2 tan α)/(1-tan^2α)=(2 cot α)/(cot^2α-1)=2/(cot α-tan α)`
`cot 2α = (cot^2α-1)/(2 cot α)=(cot α-tan α)/2=(1-tan^2α)/(2 tan α)`