﹙1/cos²80-3/cos²10﹚·1/cos20
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![﹙1/cos²80-3/cos²10﹚·1/cos20](/uploads/image/z/675157-13-7.jpg?t=%EF%B9%991%EF%BC%8Fcos%26%23178%3B80%EF%BC%8D3%EF%BC%8Fcos%26%23178%3B10%EF%B9%9A%C2%B71%EF%BC%8Fcos20)
﹙1/cos²80-3/cos²10﹚·1/cos20
﹙1/cos²80-3/cos²10﹚·1/cos20
﹙1/cos²80-3/cos²10﹚·1/cos20
(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