sin(派/4-a)=5/13
cos(派/4-a)=12/13
sin(派/2-2a)=2*5/13*12/13=120/169=cos2a
cos(派/4+a)=sin(派/2-派/4-a)=sin(派/4-a)=5/13
cos2a/[cos(派/4+a)]
=120/169÷5/13
=24/13