1-cos2a/sinacosa=1-2(cos2a/2sinacosa)
=1-2(cos2a/sin2a)
=1-2cot2a
tana=0.5,tan2a=2tana/(1-tana*tana)=4/3
cot2a=3/4
所以,1-cos2a/sinacosa=1-3/2=-1/2