=(sinx/cosx+cosx/sinx)*cos^2x
=(sin^2x+cos^2x)/cosxsinx * cos^2x
=1/sin * cosx
=cosx/sinx=cotx
^2 = 2次方.选择A是tansB是sinxC是cosxD是tanx分之一。。。。。。cot tan 互为倒数..cotx=1/tanx