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