4/3－7/12+9/20－11/30+13/42－15/56=[4/3-7/12]+[9/20-11/30]+[13/42-15/56]=3/4+1/12+1/24=3/4+1/8=7/8
{1/3*16/4-1/3*7/4}+{1/2*27/30-1/2*22/30}+{1/7*52/24-1/7*45/24}=3/4+1/12+1/24=(18+2+1)/24=7/8
