sin20=sin(30-10) 所以 (2cos10°—sin20°)/sin70° =(2cos10°—sin（30-10)）/sin70° =(2cos10-(sin30cos10-cos30sin10))/sin70 =(2cos10-1/2cos10+根号3/2sin10)/sin70 =(3/2sin10+根号3/2sin10)/sin70 =根号3...