cos65π/6=cos(65π/6-10π)=cos(5π/6)=cos(π-π/6)=-cosπ/6=-√3/2
sin(-31π/4)=sin(-31π/4+8π)=sin(π/4)=√2/2
tan(-26π/3)=tan(-26π/3+9π)=tan(π/3)=√3
sin(670°39')=sin(670°39'-720°)=sin(-49°21')=-sin49.35°=-0.7587