(1/cos280°-3/cos210°)1/cos20°中第4步怎么来的

(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