Вычислите с формул : а) cos 5п\3 б) sin(-11п\6)

А) сos 5π/3= cos (6π-π)/3=cos (6π/3-π/3)=cos(2π-π/3)=cos π/3=1/2

б) sin (-11π/6)=-sin(11π/6)=-sin(12π-π)/6= - sin(12π/6-π/6)=-sin(2π-π/6)=-(-sin π/6)=sin π/6=1/2