a=

1

2

cos60-

3

2

sin60=cos(60° +6°)=cos54°

b=

2tan130

1+tan2130

=

2sin13° cos13°

1+ sin213° cos213

=

2sin13°cos13°

1

=sin26°=cos74°

c=

1-cos500 2 cos225

=cos25

因为y=cosx在[0，

π

2

]上单调递减，所以b＜a＜c

故选B＜a＜c