cos(9x/11)-1/2=0 cos(9x/11)=1/2 9x/11 = +-arccos(1/2) +2пk 9x/11 = +-п/3 + 2пk 9x = +-11п/3 + 22пk x = +-11п/27 + 22пk/9 - нули функции -1<=cos(9x/11)<=1 -1.5<=cos(9x/11)-1/2<=0.5 Ответ: x=+-11п/27 + 22пk/9 ; [-1.5;0.5]