sin(θ-45°)=12/13,135°

cos(a)=cos((a-45)+45)=cos(a-45)*cos45-sin(a-45)*sin45
135

cosa=cos[(a-45)+45]=cos(a-45)cos45-sin(a-45)sin45=[cos(a-45)-sin(a-45)]*(#2/2),#指开根号
90cos(a-45)=-#{1-[sin(a-45)]^2}=-5/13
cosa=（-17/13)*(#2/2)