化简cos20/cos70-2cos10=cos20/(cos70-2cos10)=?
问题描述:
化简cos20/cos70-2cos10=
cos20/(cos70-2cos10)=?
答
(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(根号3/2cos10+1/2sin10)/sin70
=根号3sin(10+60)/sin70
=根号3
答
cos20/(cos70-2cos10)=cos20/(cos(60+10)-2cos10))=-根号3/3