5/7-(5/7-13/20) =5/7-　5/7　＋　13/20＝13/20.(2/3-5/6)*36=(2/3)*36 - (5/6)*36=24 - 30=-6或者：(1/3-5/6)*36=(1/3)*36 - (5/6)*36=12 - 30=-18.7/9*5/7-7/9/7=(7/9)*(5/7) - (7/9)*(1/7)=(7/9)*(4/7)=4/9...