π/4<β<α<3π/4=>π/2<α+β<3π/2=>cos(α+β)=-4/5
0<α-β<π/2=>sin(α-β)=5/13
sin(2α)=sin[(α+β)+(α-β)]
=sin(α+β)cos(α-β)+cos(α+β)sin(α-β)
=(-3/5)*(12/13)+(-4/5)*(5/13)
=-36/65-20/65
=-56/65