tg^2x=(1-sin x)(1-cos x)
tg^2x =1-sin^2x
tg^2x=cos^2x
sin^2x/cos^2x=cos^2x
sin^2x=cos^4x
1-cos^2x=cos^4x
cos^2x=t
0<=t<=1
1-t=t^2
t^2+t-1=0
D=1+4=5
t1=(-1+sqrt 5)/2
t2=(-1-sqrt 5)/2<-1
cos^2x=(-1+sqrt 5)/2
cosx=+-(sqrt(5)-1)/2
x=+-arccos[+-(sqrt(5)-1)/2]+2πn, n є Z
