1-cos2(a)=cos^2(a)+sin^2(a)-cos^2(a)+sin^2(a)=2sin^2(a)
1+cos2(а)=cos^2(a)+sin^2(a)+cos^2(a)-sin^2(a)=2cos^2(a)
2sin^2(a)/2cos^2(a)=tg^2(а)
