2tg(a-5п)+ctg(a+п/2), cos=5/13

2tga-tga=tga=sina/cosa=+-12/13;5/13=+-12/13*13/5=+-12/5cosa=5/13sina=+-√(1-cos²a)=+-√(1-25/169)=+-√(144/169)=+-12/13