sin13°cos17°-cos13sin17=1/2 sin140°cos20+sin50sin
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sin13°cos17°-cos13sin17=1/2 sin140°cos20+sin50sin
sin13°cos17°-cos13sin17=1/2
sin140°cos20+sin50sin20=√3/2
判断那个式子正确
sin13°cos17°-cos13sin17=1/2
sin140°cos20+sin50sin20=√3/2
判断那个式子正确
![sin13°cos17°-cos13sin17=1/2 sin140°cos20+sin50sin](/uploads/image/z/19843708-4-8.jpg?t=sin13%C2%B0cos17%C2%B0-cos13sin17%3D1%2F2+sin140%C2%B0cos20%2Bsin50sin)
(1)sin13°cos17°-cos13sin17=1/2.不对
因为sin13°cos17°-cos13sin17=sin(13°-17°)=-sin4°≠1/2
(2)sin140°cos20+sin50sin20
=sin40°cos20+sin50sin20
=cos50°cos20+sin50sin20
=cos(50°-20°)
=cos30°
=√3/2.对
因为sin13°cos17°-cos13sin17=sin(13°-17°)=-sin4°≠1/2
(2)sin140°cos20+sin50sin20
=sin40°cos20+sin50sin20
=cos50°cos20+sin50sin20
=cos(50°-20°)
=cos30°
=√3/2.对
利用和(差)角公式求下列各式的值:(1)sin13°cos17°+cos17°sin17°;cos80°cos20°+s
求cos20°cos10
sin40°*(1-sin10°)/[sin10°*(cos20°)^2]
化简根号下1-2sin20°cos20°
1/cos20°-根号3/sin20°
化简:sin50º(1+√3tan10°)-cos20°/cos80°√(1-cos20°)
求值:cos20°sin20°
A=(根号2)/2·(sin17°+cos17°),B=2cos²13°-1,C=(根号3)/2 排列ABC的
2sin20°cos20°-2cos²25°
计算cos20°cos40°cos80°=多少
化简:cos20°cos35°1−sin20°
求值:(2sin80°-cos70°)/cos20°