sin4π / 7⋅cos31π / 42-cos4π / 7⋅sin31π / 42=sin(4π / 7-31π / 42)=
sin(28π / 42-31π / 42)=sin(-3π / 42)=-sin(π / 14)≈-0.2894281707
sin4π / 7⋅cos31π / 42-cos4π / 7⋅sin31π / 42=sin(4π / 7-31π / 42)=
sin(28π / 42-31π / 42)=sin(-3π / 42)=-sin(π / 14)≈-0.2894281707