1-5/12+4/15+1/12-14/15-1又19/20=1-(5/12-1/12)+(4/15-14/15)-1又19/20=1-4/12-10/15-19/20=1-1/3-2/3-1又19/20=-1又19/20
1-5/12+4/15+1/12-14/15-1又19/20=1-5/12+1/12+4/15-14/15-1又19/20=1-1/3-2/3-1又19/20=1-(1/3+2/3)-1又19/20=-1又19/20=-39/20