（1）cosπ/3=cos(π/2-π/6)=sinπ/6（2）sin(3π/5)=sin(π-2/5π)=sin(2/5π)=sin(π/2-π/10)=cosπ/10（3）1920°=32/3πsin(-1920°)=sin(-32/3π)=-sin(10π+2/3π)=-sin(2/3π)=-sin(π/3)=-cos(π/6)