Question

cos^2x-2cosx-1=0 помогите пожалуйста

Answer 1

$cos^2x=t$

$t^2-2t-1=0$

$t= frac{-(-2)+- sqrt{(-2)^2}-4*1(-1) }{2*1} = frac{2+- sqrt{4+4} }{2}$

t$t= frac{2+2 sqrt{2} }{2} = frac{2(1+ sqrt{2} )}{2} =1+ sqrt{2}$

$t= frac{2-2 sqrt{2} }{2} = frac{2(1- sqrt{2} )}{2} =1- sqrt{2}$