sin470°=sin(450°+20°)=cos20°
cos110°=cos(90°+20°)=-sin20°
sin250°=sin(270°-20°)=-cos20°
cos790°=cos(810°-20°)=sin20°
所以√(1+2sin470°cos110°)/(sin250°+cos790°)
=√(1-2sin20°cos20°)/(sin20°-cos20°)
=√(sin20°-cos20°)²/(sin20°-cos20°)
=(cos20°-sin20°)/(sin20°-cos20°)
=-1