Cosx=t
|t|<=1
2t^2-3t+1=0
t1=1
t2=1/2
cosx=1
x=2Пk
cosx=1/2
x=+-П/3+2Пk
16*cos20°*cos40°*cos80°*sin20°=
=8*(2*sin20°*cos20°)*cos40°*cos80°=8*sin(2*20°)*cos40°*cos80°=
=4*(2*sin40°*cos40°)*cos80°=4sin80°*cos80°=2*(2sin80°*cos80°)=2*sin160°
L=2πR
15/60 *L =1/4 *2πR= 1/4 * 2π*3,06= 1,56(м)