Question

CosX=-1/2 чему равно?

Answer 1

Вот так: 
cosx = - 1/2
x = ± arccos (-1/2) + 2pik
x = ± ( pi - arccos (1/2)) + 2pik 
x = ± ( pi - pi/3 ) + 2pik
x = ± (3pi/3 - pi/3) + 2pik
x = ± 2pi/3 + 2pik, k  ∈Z 

Answer 2

$cosx=- \frac{1}{2} \\ x=+-arccos(- \frac{1}{2} )+2 \pi n\\x=+-( \pi -arccos\frac{1}{2})+2 \pi n\\x=+-( \pi - \frac{ \pi }{3} )+2 \pi n \\ x=+- \frac{2 \pi }{3} +2 \pi n$
n∈Z