(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的(1/cos^280-3/cos^210)*(1/cos20)=(1/cos^2(90-10)-3/cos^210)(1/cos20)=(1/sin^210-3/cos^210)*(1/cos20)=(cos^210-3sin^210)/(sin^210*cos^210)*(1/cos20)=(4cos^210-3)/(sin^210*cos^210)*(1/cos20)=(2cos20-
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![(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的(1/cos^280-3/cos^210)*(1/cos20)=(1/cos^2(90-10)-3/cos^210)(1/cos20)=(1/sin^210-3/cos^210)*(1/cos20)=(cos^210-3sin^210)/(sin^210*cos^210)*(1/cos20)=(4cos^210-3)/(sin^210*cos^210)*(1/cos20)=(2cos20-](/uploads/image/z/10349269-61-9.jpg?t=%281%2Fcos280%C2%B0-3%2Fcos210%C2%B0%291%2Fcos20%C2%B0%E4%B8%AD%E7%AC%AC4%E6%AD%A5%E6%80%8E%E4%B9%88%E6%9D%A5%E7%9A%84%281%2Fcos%5E280-3%2Fcos%5E210%29%2A%281%2Fcos20%29%3D%281%2Fcos%5E2%2890-10%29-3%2Fcos%5E210%29%281%2Fcos20%29%3D%281%2Fsin%5E210-3%2Fcos%5E210%29%2A%281%2Fcos20%29%3D%28cos%5E210-3sin%5E210%29%2F%28sin%5E210%2Acos%5E210%29%2A%281%2Fcos20%29%3D%284cos%5E210-3%29%2F%28sin%5E210%2Acos%5E210%29%2A%281%2Fcos20%29%3D%282cos20-)
(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的(1/cos^280-3/cos^210)*(1/cos20)=(1/cos^2(90-10)-3/cos^210)(1/cos20)=(1/sin^210-3/cos^210)*(1/cos20)=(cos^210-3sin^210)/(sin^210*cos^210)*(1/cos20)=(4cos^210-3)/(sin^210*cos^210)*(1/cos20)=(2cos20-
(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的
(1/cos^280-3/cos^210)*(1/cos20)
=(1/cos^2(90-10)-3/cos^210)(1/cos20)
=(1/sin^210-3/cos^210)*(1/cos20)
=(cos^210-3sin^210)/(sin^210*cos^210)*(1/cos20)
=(4cos^210-3)/(sin^210*cos^210)*(1/cos20)
=(2cos20-1)/(sin^210*cos^210)*(1/cos20)
=4(2cos20-1)/(sin^210*cos^210)*(1/cos20)
=8 (cos20 - cos60 ) / sin^220 *(1/cos20)
=16sin40sin20 / sin^220 *(1/cos20)
=32 cos20 *(1/cos20)
=32
(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的(1/cos^280-3/cos^210)*(1/cos20)=(1/cos^2(90-10)-3/cos^210)(1/cos20)=(1/sin^210-3/cos^210)*(1/cos20)=(cos^210-3sin^210)/(sin^210*cos^210)*(1/cos20)=(4cos^210-3)/(sin^210*cos^210)*(1/cos20)=(2cos20-
因为sin²10°=1-cos²10°
带入第三步就得到第四步