Question

Как найти –cosx если cosx/2=√0,8

Answer 1

$-cosx=-cos(2* \frac{x}{2} )=-(cos^2( \frac{x}{2} )-sin^2( \frac{x}{2} ))= \\ \\ =sin^2( \frac{x}{2}) -cos^2( \frac{x}{2} )=1-cos^2( \frac{x}{2} )-cos^2( \frac{x}{2} )= \\ \\ =1-2cos^2( \frac{x}{2} ) \\ \\ 1-2( \sqrt{0.8} )^2=1-2*0.8=1-1.6=-0.6$