Tg^2x=(1-sin x)(1-cos x) tg^2x =1-sin^2x

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