Cos2α=cos(α+α)
=cosαcosα-sinαsinα
=cos^2α-sin^2α
=1-sin^2α-sin^2α
=1-2Sinα^2
Cos2α=cos(α+α)
=cosαcosα-sinαsinα
=cos^2α-sin^2α
=cos^2α-(1-cos^2α)
=2cos^2α-1