(3/4-4/5)*7,8+(2/3+4/7)*7/13=83/300
3,1(28)+2,(21)=3,1(28)+2,2(12)=4,3(40)
sin2pix=1
2pix=pi/2+2pik
x=k+1/4
4x-1<>0
4x<>1
x<>1/4
k+1/4<>1/4
k<>0