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

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