原式=－311/980
2/7×(5/4+5/7)－3/4=2/7×5/4+2/7×5/7－3/4=5/14+10/49－3/4=(5×7×2)/(14×7×2)+(10×4)/(49×4)－(3×49)/(4×49)=70/196+40/196－147/196=-37/196
